fix visitShift to properly zero extend the shift amount if the provided operand

fix visitShift to properly zero extend the shift amount if the provided operand
is narrower than the shift register.  Doing an anyext provides undefined bits in
the top part of the register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125457 91177308-0d34-0410-b5e6-96231b3b80d8