[mips] Restrict the creation of compact branches

Restrict the creation of compact branches so that they meet the ISA encoding
requirements. Notably do not permit $zero to be used as a operand for compact
branches and ensure that some other branches fulfil the requirement that
rs != rt.

Fixup cases where $rs > $rt for bnec and beqc.

Reviewers: dsanders, vkalintiris

Differential Review: http://reviews.llvm.org/D20284

llvm-svn: 269893