compile time improvement - add __zig_panic_slice fn

move some boilerplate code having to do with panicking
to a function.

Here's the timing difference. It's not much:

Before:

full test:  1m36.511s
debug test: 20.862s
hello.zig
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0001
   Semantic Analysis      0.0000      0.0421      0.0420      0.2109
     Code Generation      0.0421      0.0620      0.0200      0.1003
    LLVM Emit Object      0.0620      0.1852      0.1231      0.6180
  Build Dependencies      0.1852      0.1974      0.0122      0.0615
           LLVM Link      0.1974      0.1993      0.0018      0.0093
         Generate .h      0.1993      0.1993      0.0000      0.0000
               Total      0.0000      0.1993      0.1993      1.0000

After:

full test:  1m33.588s
debug test: 20.303s
hello.zig
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0002
   Semantic Analysis      0.0000      0.0425      0.0425      0.2202
     Code Generation      0.0425      0.0675      0.0250      0.1293
    LLVM Emit Object      0.0675      0.1789      0.1114      0.5773
  Build Dependencies      0.1789      0.1913      0.0124      0.0640
           LLVM Link      0.1913      0.1931      0.0018      0.0091
         Generate .h      0.1931      0.1931      0.0000      0.0000
               Total      0.0000      0.1931      0.1931      1.0000

src/all_types.hpp

@@ -1460,6 +1460,7 @@ struct CodeGen {
     LLVMValueRef err_name_table;
     size_t largest_err_name_len;
     LLVMValueRef safety_crash_err_fn;
+    LLVMValueRef panic_slice_fn;
 
     IrInstruction *invalid_instruction;
     ConstExprValue const_void_val;

src/codegen.cpp

@@ -647,8 +647,28 @@ static void gen_panic_raw(CodeGen *g, LLVMValueRef msg_ptr, LLVMValueRef msg_len
     LLVMBuildUnreachable(g->builder);
 }
 
-static void gen_panic(CodeGen *g, LLVMValueRef msg_arg) {
+static LLVMValueRef get_panic_slice_fn(CodeGen *g) {
+    if (g->panic_slice_fn != nullptr)
+        return g->panic_slice_fn;
+
     TypeTableEntry *str_type = get_slice_type(g, g->builtin_types.entry_u8, true);
+    TypeTableEntry *ptr_to_str_type = get_pointer_to_type(g, str_type, true);
+
+    Buf *fn_name = get_mangled_name(g, buf_create_from_str("__zig_panic_slice"), false);
+    LLVMTypeRef fn_type_ref = LLVMFunctionType(LLVMVoidType(), &ptr_to_str_type->type_ref, 1, false);
+    LLVMValueRef fn_val = LLVMAddFunction(g->module, buf_ptr(fn_name), fn_type_ref);
+    addLLVMFnAttr(fn_val, "noreturn");
+    addLLVMFnAttr(fn_val, "cold");
+    LLVMSetLinkage(fn_val, LLVMInternalLinkage);
+    LLVMSetFunctionCallConv(fn_val, LLVMFastCallConv);
+
+    LLVMBasicBlockRef entry_block = LLVMAppendBasicBlock(fn_val, "Entry");
+    LLVMBasicBlockRef prev_block = LLVMGetInsertBlock(g->builder);
+    LLVMValueRef prev_debug_location = LLVMGetCurrentDebugLocation(g->builder);
+    LLVMPositionBuilderAtEnd(g->builder, entry_block);
+
+    LLVMValueRef msg_arg = LLVMGetParam(fn_val, 0);
+
     size_t ptr_index = str_type->data.structure.fields[slice_ptr_index].gen_index;
     size_t len_index = str_type->data.structure.fields[slice_len_index].gen_index;
     LLVMValueRef ptr_ptr = LLVMBuildStructGEP(g->builder, msg_arg, (unsigned)ptr_index, "");
@@ -657,6 +677,17 @@ static void gen_panic(CodeGen *g, LLVMValueRef msg_arg) {
     LLVMValueRef msg_ptr = LLVMBuildLoad(g->builder, ptr_ptr, "");
     LLVMValueRef msg_len = LLVMBuildLoad(g->builder, len_ptr, "");
     gen_panic_raw(g, msg_ptr, msg_len);
+
+    LLVMPositionBuilderAtEnd(g->builder, prev_block);
+    LLVMSetCurrentDebugLocation(g->builder, prev_debug_location);
+    g->panic_slice_fn = fn_val;
+    return g->panic_slice_fn;
+}
+
+static void gen_panic(CodeGen *g, LLVMValueRef msg_arg) {
+    LLVMValueRef fn_val = get_panic_slice_fn(g);
+    LLVMBuildCall(g->builder, fn_val, &msg_arg, 1, "");
+    LLVMBuildUnreachable(g->builder);
 }
 
 static void gen_debug_safety_crash(CodeGen *g, PanicMsgId msg_id) {