[LoongArch] Revert `sp` adjustment in prologue (#88110)

After commit 18c5f3c3 ("[RegisterScavenger][RISCV] Don't search for
FrameSetup instrs if we were searching from Non-FrameSetup instrs"), we
can revert the `sp` adjustment 4e2364a2 ("[LoongArch] Add emergency
spill slot for GPR for large frames") to generate better code, as the
issue with `RegScavenger` has been resolved.

Fixes #88109