[InstCombine] Fold integer unpack/repack patterns through ZExt (#153583)
This patch explicitly enables the InstCombiner to fold integer
unpack/repack patterns such as
```llvm
define i64 @src_combine(i32 %lower, i32 %upper) {
%base = zext i32 %lower to i64
%u.0 = and i32 %upper, u0xff
%z.0 = zext i32 %u.0 to i64
%s.0 = shl i64 %z.0, 32
%o.0 = or i64 %base, %s.0
%r.1 = lshr i32 %upper, 8
%u.1 = and i32 %r.1, u0xff
%z.1 = zext i32 %u.1 to i64
%s.1 = shl i64 %z.1, 40
%o.1 = or i64 %o.0, %s.1
%r.2 = lshr i32 %upper, 16
%u.2 = and i32 %r.2, u0xff
%z.2 = zext i32 %u.2 to i64
%s.2 = shl i64 %z.2, 48
%o.2 = or i64 %o.1, %s.2
%r.3 = lshr i32 %upper, 24
%u.3 = and i32 %r.3, u0xff
%z.3 = zext i32 %u.3 to i64
%s.3 = shl i64 %z.3, 56
%o.3 = or i64 %o.2, %s.3
ret i64 %o.3
}
; =>
define i64 @tgt_combine(i32 %lower, i32 %upper) {
%base = zext i32 %lower to i64
%upper.zext = zext i32 %upper to i64
%s.0 = shl nuw i64 %upper.zext, 32
%o.3 = or disjoint i64 %s.0, %base
ret i64 %o.3
}
```
Alive2 proofs: [YAy7ny](https://alive2.llvm.org/ce/z/YAy7ny)
82caa251d4