[XCore] Don't create invalid MKMSK instructions inside loadImmediate().

Summary:
Previously loadImmediate() would produce MKMSK instructions with invalid
immediate values such as mkmsk r0, 9. Fix this by checking the mask size
is valid.

Reviewers: robertlytton

Reviewed By: robertlytton

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3289

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206163 91177308-0d34-0410-b5e6-96231b3b80d8