[llvm][PowerPC] Correct handling of spill slots for SPE when EXPENSIVE_CHECKS is enabled (#73940)

This was modifying a container as it iterated it, which tripped a check
in libstdc++'s debug checks.

Instead, just assign to the item via the reference we already have.

This fixes the following expensive checks failures on my machine:
  LLVM :: CodeGen/PowerPC/fp-strict.ll
  LLVM :: CodeGen/PowerPC/pr55463.ll
  LLVM :: CodeGen/PowerPC/register-pressure.ll
  LLVM :: CodeGen/PowerPC/spe.ll

Which are some of the tests noted by #68594.