[InstCombine] Canonicalise packed-integer-selecting shifts (#162147)

This patch resolves recent regressions related to [issue
#92891](https://github.com/llvm/llvm-project/issues/92891).
It specifically enables the following types of reductions.

```llvm
define i16 @src(i32 %mask, i32 %upper, i32 range(i32 0, 65536) %lower) {
  %upper.shl = shl nuw i32 %upper, 16
  %pack = or disjoint i32 %upper.shl, %lower
  %mask.bit = and i32 %mask, 16
  %sel = lshr i32 %pack, %mask.bit
  %trunc = trunc i32 %sel to i16
  ret i16 %trunc
}
; =>
define i16 @tgt(i32 %mask, i32 %upper, i32 range(i32 0, 65536) %lower) {
  %mask.bit = and i32 %mask, 16
  %mask.bit.z = icmp eq i32 %mask.bit, 0
  %sel = select i1 %mask.bit.z, i32 %lower, i32 %upper
  %trunc = trunc i32 %sel to i16
  ret i16 %trunc
}
```

Alive2 proofs: [gJ9MpP](https://alive2.llvm.org/ce/z/gJ9MpP)

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