[RISCV] For rv32, accept constants like 0xfffff800 as a valid simm12.

Internally we store constants in int64_t after parsing, but this is
kind of an implementation detail. If we only supported rv32, we might
have chosen int32_t.

For rv32, I think it makes sense to accept the constants that we
would accept if int32_t was the internal type. In fact we already
do this for the `li` alias. This patch extends this to sign
extended constants for other instructions.

This matches the GNU assembler. The difference between LLVM and gcc
was previously noted here. https://github.com/riscv-non-isa/riscv-asm-manual/pull/71

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D144166