[LoongArch] Add emergency spill slot for GPR for large frames

An emergency spill slot is created when the stack size cannot be
represented by an 11-bit signed number.

This patch also modifies how the `sp` is adjusted in the prologue.

`RegScavenger` will place the spill instruction before the prologue
if a VReg is created in the prologue. This will pollute the caller's
stack data. Therefore, until there is better way, we just use the
`addi.w/d` instruction for stack adjustment to ensure that VReg will
not be created. (RISCV has the same issue #58286)

Due to the addition of emergency spill slot, some test cases that use
exact stacksize need to be updated.

Differential Revision: https://reviews.llvm.org/D135757