[RISCV] Refactor and improve eliminateFrameIndex.

There are few changes mixed in here.

-Try to reuse the destination register from ADDI instead of always
creating a virtual register. This way we lean on the register
scavenger in fewer case.
-Explicitly reuse the primary virtual register when possible. There's
still a case where both getVLENFactoredAmount and handling large
fixed offsets can both create a secondary virtual register.
-Combine similar BuildMI calls by manipulating the Register variables.

There are still a couple early outs for ADDI, but overall I tried to
arrange the code into steps.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D135009