[Mips] Make MipsBranchExpansion aware of BBIT family of branch

Octeon branches (bbit0/bbit032/bbit1/bbit132) have an immediate operand,
so it is legal to have such replacement within
MipsBranchExpansion::replaceBranch().

According to the specification, a branch (e.g. bbit0 ) looks like:

bbit0  rs p offset  // p is an immediate operand
  if !rs<p> then branch

Without this patch, an assertion triggers in the method,
and the problem has been found in the real example.

Differential Revision: https://reviews.llvm.org/D76842