Broken deminance in produced llvm ir

[espindola]

Given the program

const std = @import("std");
const fs = std.fs;
const io = std.io;
const PT_TLS = 7;

const PT_LOPROC = 0x70000000;
const PT_HIPROC = 0x7fffffff;

fn printProgramHeaderTypeName(t: u32, stdout: fs.File.Writer) !void {
    try switch (t) {
        PT_TLS => stdout.writeAll("foo"),
        PT_LOPROC...PT_HIPROC => stdout.print("bar", .{}),
        else => {
            return switch (t) {
                else => stdout.print("xyz", .{}),
            };
        },
    };
}

pub fn main() !void {
    const stdout = io.getStdOut().writer();
    try printProgramHeaderTypeName(42, stdout);
}

A debug build of zig at 11ae6c4 produces invalid LLVM IR, resulting in the error:

broken LLVM module found: Instruction does not dominate all uses!td.os.WriteError,st...
  %12 = call fastcc i16 @"std.io.writer.Writer(std.fs.file.File,std.os.WriteError,std.fs.file.File.write).print.166"(%std.builtin.StackTrace* %0, %"std.io.writer.Writer(std.fs.file.File,std.os.WriteError,std.fs.file.File.write)"* %2), !dbg !18191
  store i16 %12, i16* %4, align 2, !dbg !18184
Instruction does not dominate all uses!
  %12 = call fastcc i16 @"std.io.writer.Writer(std.fs.file.File,std.os.WriteError,std.fs.file.File.write).print.166"(%std.builtin.StackTrace* %0, %"std.io.writer.Writer(std.fs.file.File,std.os.WriteError,std.fs.file.File.write)"* %2), !dbg !18191
  %13 = icmp ne i16 %12, 0, !dbg !18192

The backtrace is:

#0  0x00007ffff787cd22 in raise () from /usr/lib/libc.so.6
#1  0x00007ffff7866862 in abort () from /usr/lib/libc.so.6
#2  0x0000555556b93269 in std.os.abort () at /home/espindola/zig/zig/lib/std/os.zig:233
#3  0x0000555556b9dcab in std.debug.panicExtra (trace=0x0, first_trace_addr=..., args=...) at /home/espindola/zig/zig/lib/std/debug.zig:305
#4  0x0000555556b8efeb in std.builtin.default_panic (msg=..., error_return_trace=0x0) at /home/espindola/zig/zig/lib/std/builtin.zig:690
#5  0x0000555556b9fe28 in stage2_panic (ptr=0x55556015b94c "", len=0) at /home/espindola/zig/zig/src/stage1.zig:171
#6  0x00005555577064bd in zig_panic (format=0x55556015d9a0 "broken LLVM module found: %s\nThis is a bug in the Zig compiler.") at ../src/stage1/util.cpp:20
#7  0x000055555772c6c5 in do_code_gen (g=0x555568805550) at ../src/stage1/codegen.cpp:8489
#8  0x00005555577314e8 in codegen_build_object (g=0x555568805550) at ../src/stage1/codegen.cpp:9641
#9  0x00005555577023cc in zig_stage1_build_object (stage1=0x555568805550) at ../src/stage1/stage1.cpp:130
#10 0x0000555556f41af5 in Module.build_object (mod=0x555568805550) at /home/espindola/zig/zig/src/stage1.zig:143
#11 0x0000555556d2de15 in Compilation.updateStage1Module (comp=0x555568803148, main_progress_node=0x7fffffff8be8) at /home/espindola/zig/zig/src/Compilation.zig:3950
#12 0x0000555556d05401 in Compilation.performAllTheWork (self=0x555568803148) at /home/espindola/zig/zig/src/Compilation.zig:2232
#13 0x0000555556cff4db in Compilation.update (self=0x555568803148) at /home/espindola/zig/zig/src/Compilation.zig:1631
#14 0x0000555556c99e60 in main.updateModule (gpa=0x5555686daef0 <c_allocator_state>, comp=0x555568803148, hook=...) at /home/espindola/zig/zig/src/main.zig:2255
#15 0x0000555556bc9033 in main.buildOutputType (gpa=0x5555686daef0 <c_allocator_state>, arena=0x7fffffffe5b8, all_args=..., arg_mode=...) at /home/espindola/zig/zig/src/main.zig:1987
#16 0x0000555556b9e398 in main.mainArgs (gpa=0x5555686daef0 <c_allocator_state>, arena=0x7fffffffe5b8, args=...) at /home/espindola/zig/zig/src/main.zig:202
#17 0x0000555556b9ddda in main (argc=3, argv=0x7fffffffe7f8) at /home/espindola/zig/zig/src/stage1.zig:45