[InstCombine] ease alignment restriction for converting masked load to normal load

I think we initially made this fold conservative to be safer, but we do not
need the alignment attribute/metadata limitation because the masked load
intrinsic itself specifies the alignment. A normal vector load is better for
IR transforms and should be no worse in codegen than the masked alternative.
If it is worse for some target, the backend can reverse this transform.

Differential Revision: https://reviews.llvm.org/D88505