[RISCV] Add additional CSR instruction aliases (imm. operands)

This patch adds CSR instructions aliases for the cases where the instruction 
takes an immediate operand but the alias doesn't have the i suffix. This is 
necessary for gas/gcc compatibility.

gas doesn't do a similar conversion for fsflags or fsrm, so this should be 
complete.

Differential Revision: https://reviews.llvm.org/D55008
Patch by Luís Marques.

llvm-svn: 347991