[mips] Trivial corrections to range checked immediates.

Summary:
SYNC has a 5-bit unsigned immediate.
Move MIPS16-specific pcrel16 operand to Mips16 files.

Reviewers: vkalintiris

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D18755

llvm-svn: 265947