[ARM GlobalISel] Select extensions to < 32 bits

[ARM GlobalISel] Select extensions to < 32 bits

Select G_SEXT and G_ZEXT with destination types smaller than 32 bits in
the exact same way as 32 bits. This overwrites the higher bits, but that
should be ok since all legal users of types smaller than 32 bits ignore
those bits anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359768 91177308-0d34-0410-b5e6-96231b3b80d8