Handle subregs and superregs in callee-saved register mask

If a target lists both a subreg and a superreg in a callee-saved
register mask, the prolog will spill both aliasing registers. Instead,
don't spill the subreg if a superreg is being spilled. This case is hit by the
PowerPC SPE code, as well as a modified RISC-V backend for CHERI I maintain out
of tree.

Reviewed By: jhibbits

Differential Revision: https://reviews.llvm.org/D73170