[mips][ias] Range check uimm6 operands and fix a bug this revealed.

Summary:
We don't check the size operand on ext/dext*/ins/dins* yet because the
permitted range depends on the pos argument and we can't check that using
this mechanism.

The bug was that dextu/dinsu accepted 0..31 in the pos operand instead of 32..63.

Reviewers: vkalintiris

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D15190

llvm-svn: 255015