Fix mapping of @llvm.arm.ssat/usat intrinsics to ssat/usat instructions

The mapping of these two intrinsics in ARMInstrInfo.td had a small
omission which lead to their operands not being validated/transformed
before being lowered into usat and ssat instructions. This can cause
incorrect instructions to be emitted.

I've also added tests for the remaining two saturating arithmatic
intrinsics @llvm.arm.qadd and @llvm.arm.qsub as they are missing
codegen tests.

llvm-svn: 250697