InstCombine calculated a wrong `insertelement` instruction

[dianqk]

I tried this IR:

define <4 x i32> @src(i32 %arg) {
  %i = zext i32 %arg to i64
  %i1 = shl i64 %i, 32
  %i2 = or i64 %i1, 65
  %i3 = zext i64 %i2 to i128
  %i4 = shl i128 %i3, 64
  %i5 = bitcast i128 %i4 to <4 x i32>
  ret <4 x i32> %i5
}

define <4 x i32> @tgt(i32 %arg) {
  %i5 = insertelement <4 x i32> <i32 0, i32 0, i32 0, i32 poison>, i32 %arg, i64 3
  ret <4 x i32> %i5
}

I expect src to be:

define <4 x i32> @src(i32 %arg) {
  %i5 = insertelement <4 x i32> <i32 0, i32 0, i32 65, i32 poison>, i32 %arg, i64 3
  ret <4 x i32> %i5
}

But I'm getting the same instructions as tgt.

Godbolt: https://llvm.godbolt.org/z/KnMEY4rTq
Alive2: https://alive2.llvm.org/ce/z/7FIJNi
From: rust-lang/rust#121996

[cbeuw]

Looks like this bug is almost 11 years old: feb3471! Thanks for fixing

[dianqk]

Looks like this bug is almost 11 years old: feb3471! Thanks for fixing

Based on my experience, we likely have many more latent bugs. :)

[dianqk]

/cherry-pick e96c0c1