[mips][ias] Range check uimm4 operands and fixed a bug this revealed.

Summary:
The bug was that the sldi instructions have immediate widths dependant on
their element size. So sldi.d has a 1-bit immediate and sldi.b has a 4-bit
immediate. All of these were using 4-bit immediates previously.

Reviewers: vkalintiris

Subscribers: llvm-commits, atanasyan, dsanders

Differential Revision: http://reviews.llvm.org/D14018

llvm-svn: 252297