[XCore] Fix instruction selection for zext, mkmsk instructions.

r182680 replaced CountLeadingZeros_32 with a template function
countLeadingZeros that relies on using the correct argument type to give
the right result. The type passed in the XCore backend after this
revision was incorrect in a couple of places.

Patch by Robert Lytton.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185430 91177308-0d34-0410-b5e6-96231b3b80d8