stage2: Runtime result of `@bitCast(u16, @Vector(16, bool))` is incorrect

[topolarity]

Zig Version

0.10.0-dev.4430+78778899

Steps to Reproduce

const std = @import("std");
const expect = std.testing.expect;

test "bitcast vector to integer and back" {
    var x = @splat(16, true);
    x[1] = false;
    try expect(@bitCast(u16, x) == 0xfffd);
}

Expected Behavior

Test should pass, like it does on stage1.

Actual Behavior

Test fails

[Vexu]

Related #12812, stage1 handles both cases correctly by using insertelement/extractelement instead of getelementptr like stage2 does:

; stage2
define dso_local i32 @main() {
Entry:
  %0 = alloca <16 x i1>, align 2
  store <16 x i1> <i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true>, ptr %0, align 2
  %1 = getelementptr inbounds <16 x i1>, ptr %0, i32 0, i64 1
  store i1 false, ptr %1, align 1
  %2 = load <16 x i1>, ptr %0, align 2
  %3 = bitcast <16 x i1> %2 to i16
  %4 = icmp ne i16 %3, -3
  br i1 %4, label %Then, label %Else

Then:
  call fastcc void @abort()
  unreachable

Else:
  ret i32 0
}

; stage1
define i32 @main() {
Entry:
  %x = alloca <16 x i1>, align 2
  store <16 x i1> <i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true, i1 true>, ptr %x, align 2
  %0 = load <16 x i1>, ptr %x, align 2
  %1 = insertelement <16 x i1> %0, i1 false, i32 1
  store <16 x i1> %1, ptr %x, align 2
  %2 = load <16 x i1>, ptr %x, align 2
  %3 = bitcast <16 x i1> %2 to i16
  %4 = icmp ne i16 %3, -3
  br i1 %4, label %Then, label %Else

Then:
  call fastcc void @abort()
  unreachable

Else:
  ret i32 0
}

[Vexu]

Fixed by #13959, test re-enabled in #14272