[mips] [IAS] Fix error messages for using LI with 64-bit immediates.

Summary:
LI should never accept immediates larger than 32 bits.
The additional Is32BitImm boolean also paves the way for unifying the functionality that LA and LI have in common.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9289

llvm-svn: 236313