[RISCV] Add scalable offset under very large stack size.

If the stack size is larger than 12 bits, we have to use a scratch
register to store the stack size. Before we introduce the scalable stack
offset, we could simplify

%0 = ADDI %stack.0, 0

=>

%scratch = ... # sequence of instructions to move the offset into
%%scratch
%0 = ADD %fp, %scratch

However, if the offset contains scalable part, we need to consider it.

%0 = ADDI %stack.0, 0

=>

%scratch = ... # sequence of instructions to move the offset into
%%scratch
%scratch = ADD %fp, %scratch
%scalable_offset = ... # sequence of instructions for vscaled-offset.
%0 = ADD/SUB %scratch, %scalable_offset

Differential Revision: https://reviews.llvm.org/D100035