[RISCV] Improve codegen for icmp {ne,eq} with a constant

[RISCV] Improve codegen for icmp {ne,eq} with a constant

Adds two patterns to improve the codegen of GPR value comparisons with small
constants. Instead of first loading the constant into another register and then
doing an XOR of those registers, these patterns directly use the constant as an
XORI immediate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356990 91177308-0d34-0410-b5e6-96231b3b80d8