[llvm] Update the base and index value for masked gather (PR #130920)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

x86 natively supports gather/scatter nodes with scale values of 1/2/4/8 (similar to regular address math). If the index node is ISD::SHL by a constant shift amount, we might be able to transfer some/all of the shift amount into the scale:
```
   //  index = index_src << { 1, 2, 3, 4 } ; min_shift_amount = 1
   if (numsignbits(index_src) > (1 + min_shift_amount) {
     newscale = oldscale * (1 << min_shift_amount) ; iff newscale <= 8
     newindex  = index_src << { 0, 1, 2, 3 } ;
  }
```
this could then result in newindex being able to be safely truncated to a vXi32 type

https://github.com/llvm/llvm-project/pull/130920