[RISCV] Adjust first sp size to use c.addi16sp.

addi sp, sp, 512 may be used to recover the sp in the epilogue
when stack size is larger than 2047(2^11 - 1), however, it can
not be compressed using C extension, and addi sp, sp, 496 is
able to be compressed, so try to use 496 as the ajust amount of
the fisrt sp if function doesn't need extra instructions after
adjust.

Reviewed By: wangpc

Differential Revision: https://reviews.llvm.org/D159431