[RISCV] Allow spilling to unused Zcmp Stack (#125959)
This is a tiny change that can save up to 16 bytes of stack allocation,
which is more beneficial on RV32 than RV64.
cm.push allocates multiples of 16 bytes, but only uses a subset of those
bytes for pushing callee-saved registers. Up to 12 (rv32) or 8 (rv64)
bytes are left unused, depending on how many registers are pushed.
Before this change, we told LLVM that the entire allocation was used, by
creating a fixed stack object which covered the whole allocation.
This change instead gives an accurate extent to the fixed stack object,
to only cover the registers that have been pushed. This allows the
PrologEpilogInserter to use any unused bytes for spills. Potentially
this saves an extra move of the stack pointer after the push, because
the push can allocate up to 48 more bytes than it needs for registers.
We cannot do the same change for save/restore, because the restore
routines restore in batches of `stackalign/(xlen/8)` registers, and we
don't want to clobber the saved values of registers that we didn't tell
the compiler we were saving/restoring - for instance `__riscv_restore_0`
is used by the compiler when it only wants to save `ra`, but will end up
restoring `ra` and `s0`.
