[RISCV][MC] Modify evaluateConstantImm interface to allow reuse from addExpr

This is a trivial refactoring that I'm committing now as it makes a patch I'm 
about to post for review easier to follow. There is some overlap between 
evaluateConstantImm and addExpr in RISCVAsmParser. This patch allows 
evaluateConstantImm to be reused from addExpr to remove this overlap. The 
benefit will be greater when a future patch adds extra code to allows 
immediates to be evaluated from constant symbols (e.g. `.equ CONST, 0x1234`).

No functional change intended.

llvm-svn: 342641