[mlir][llvm] Introduce some constant folding.

This revision introduces some constant folding features to the LLVM
dialect. This specific choice of operations to cover is intended to
allow the elimination of logic generated by mem2reg with memset in the
common case of memsets of constant values.

This also introduces new verifiers for integer extension operations.
This lead to a fix in SPIRV to LLVM conversion, as it would sometimes
generate invalid ZExt and SExt operations.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D153135