[RISCV] Optimize immediate materialisation with SLLI.UW in the Zba extension

Simplify "LUI+SLLI+ADDI+SLLI" and "LUI+ADDIW+SLLI+ADDI+SLLI" to
"LUI+ADDIW+SLLIUW" to reduce total instruction amount.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D111933