[LegalizeTypes][RISCV] Correctly sign-extend comparison for ATOMIC_CMP_XCHG

Currently, the comparison argument used for ATOMIC_CMP_XCHG is legalised
with GetPromotedInteger, which leaves the upper bits of the value
undefind. Since this is used for comparing in an LR/SC loop with a
full-width comparison, we must sign extend it on RISC-V.

This is related to https://reviews.llvm.org/D58829, which solved the
issue for ATOMIC_CMP_SWAP_WITH_SUCCESS, but not the simpler
ATOMIC_CMP_SWAP.

This patch is a modified form of
616289ed by Jessica Clarke. It localises
the changes to LegalizeIntegerTypes and avoids adding a new virtual
method to TargetLowering to avoid changing the ABI of libLLVM.so.