[GlobalISel]: Create VREGs for ConstantInt args

This patch changes the behavior of IRTranslating intrinsics where we
now create VREG + G_CONSTANT for ConstantInt values. We already do this
for FloatingPoint values. This makes it easier for the backends to
select code and it won't have to de-duplicate creation+selection of
constants.

Reviewed by: ab

llvm-svn: 298473