Stop APInt::shl from generating llvm.trap

APInt::shl generated llvm.trap to guard against shifts greater than bit-width.
This was already checked with an assert, and there was a special case for
shifts equal to bit-width. Modify this check to catch shifts greater than or
equal to bit-width, so llvm.trap isn't generated.

Patch contributed by JF Bastien

llvm-svn: 166803