[RISCV] Improve lowering of SHL_PARTS/SRL_PARTS/SRA_PARTS.

Part of the shift lowering creates a (sub XLEN-1, ShAmt). When this
value is used we know that ShAmt is [0..XLEN-1]. Since XLEN is a power
of 2 we can replace the sub with an xor. This allows us to use XORI
instead of LI+SUB.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D119411