Do not emit undefined lsrh/ashr for NEON shifts

These IR instructions are undefined when the amount is equal to operand
size, but NEON right shifts support such shifts. Work around that by
emitting a different IR in these cases.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191953 91177308-0d34-0410-b5e6-96231b3b80d8