[llvm] [Reland][ValueTracking] Improve Bitcast handling to match SDAG (PR #145223)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

shouldn't you be folding this? the zeroinitializer looks like an excess reduction that may be screwing things up 
```
define fastcc <2 x i64> @_mm_srai_epi16(<2 x i64> %0) #0 {
  %2 = bitcast <2 x i64> %0 to <8 x i16>
  %3 = ashr <8 x i16> %2, splat (i16 1)
  ret <2 x i64> %3
}
```

https://github.com/llvm/llvm-project/pull/145223