We can handle the following cases + some `nsw`/`nuw` flags:

`(srem (mul X, Y), (mul X, Z))`

[If `srem(Y, Z) == 0`] -> 0 - https://alive2.llvm.org/ce/z/PW4XZ- [If `srem(Y, Z) == Y`] -> `(mul nuw nsw X, Y)` - https://alive2.llvm.org/ce/z/DQe9Ek -> `(mul nsw X, Y)` - https://alive2.llvm.org/ce/z/Nr_MdH [If `Y`/`Z` are constant] -> `(mul/shl nuw nsw X, (srem Y, Z))` - https://alive2.llvm.org/ce/z/ccTFj2 - https://alive2.llvm.org/ce/z/i_UQ5A -> `(mul/shl nsw X, (srem Y, Z))` - https://alive2.llvm.org/ce/z/mQKc63 - https://alive2.llvm.org/ce/z/uERkKH

`(urem (mul X, Y), (mul X, Z))`

[If `urem(Y, Z) == 0`] -> 0 - https://alive2.llvm.org/ce/z/LL7UVR [If `srem(Y, Z) == Y`] -> `(mul nuw nsw X, Y)` - https://alive2.llvm.org/ce/z/9Kgs_i -> `(mul nuw X, Y)` - https://alive2.llvm.org/ce/z/ow9i8u [If `Y`/`Z` are constant] -> `(mul nuw nsw X, (srem Y, Z))` - https://alive2.llvm.org/ce/z/mNnQqJ - https://alive2.llvm.org/ce/z/Bj_DR- - https://alive2.llvm.org/ce/z/X6ZEtQ -> `(mul nuw X, (srem Y, Z))` - https://alive2.llvm.org/ce/z/SJYtUV

The rationale for doing this all in `InstCombine` rather than handling

the constant `mul` cases in `InstSimplify` is we often create a new

instruction because we are able to deduce more `nsw`/`nuw` flags than

the original instruction had.

A Variety of transforms. This comment can be moved above the function.