[AArch64][GlobalISel] Add lowering for s/umul.fix.sat (#217221)

This extends the existing s/umul.fix lowering to handle the saturating
versions of the instructions. They use a TruncSSatS or TruncUSatU
instead of the trunc, keeping the implementation simple. That does
require truncsat lowering to be added to allow those operations to
subsequently lower if needed.

GitOrigin-RevId: ab9c222cf9ccec462fab80846134d96f7e400396