[mips] Range check uimm20 and fixed a bug this revealed.

Summary:
The bug was that dextu's operand 3 would print 0-31 instead of 32-63 when
printing assembly. This came up when replacing
MipsInstPrinter::printUnsignedImm() with a version that could handle arbitrary
bit widths.

MipsAsmPrinter::printUnsignedImm*() don't seem to be used so they have been
removed.

Reviewers: vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D15521

llvm-svn: 262231