[PATCH] [mips] Restrict the creation of compact branches

Restrict the creation of compact branches so that they do meet the ISA
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.

Recommit of rL269893 with reviewers comments.

Reviewers: dsanders, vkalintiris

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

llvm-svn: 269899