[RISCV] Teach shouldConvertConstantLoadToIntImm that constant materialization...

[RISCV] Teach shouldConvertConstantLoadToIntImm that constant materialization can use constant pools.

I think it only makes sense to return true here if we aren't going
to turn around and create a constant pool for the immmediate.

I left out the check for useConstantPoolForLargeInts() thinking
that even if you don't want the commpiler to create a constant pool
you might still want to avoid materializing an integer that is
already available in a global variable.

Test file was copied from AArch64/ARM and has not been commited yet.
Will post separate review for that.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D129402