GlobalISel: Fix narrowScalar for load/store with different mem size

GlobalISel: Fix narrowScalar for load/store with different mem size

This was ignoring the memory size, and producing multiple loads/stores
if the operand size was different from the memory size.

I assume this is the intent of not having an explicit G_ANYEXTLOAD
(although I think that would probably be better).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352523 91177308-0d34-0410-b5e6-96231b3b80d8