[llvm] [LLVM][DAGCombiner] Improve simplifyDivRem's effectiveness after type legalisation. (PR #162706)

[Hari Limaye via llvm-commits]

hazzlim wrote:

> simplifyDivRem does not work as well after type legalsiation because splatted constants can have a size mismatch between the scalar to splat and the element type of the splatted result. simplifyDivRem does not seem to care about this mismatch so I've updated the isConstOrConstSplat call for the divisor to allow truncation.
> 
> This comes from investigating #162616. I have a separate fix for the incorrect lowering but I figure this should not have got through to lowering in the first place.

Commit message typo nit: legalsiation -> legalization/legalisation

https://github.com/llvm/llvm-project/pull/162706