[MIPS GlobalISel] Select G_FENCE

G_FENCE comes form fence instruction. For MIPS fence is generated in
AtomicExpandPass when atomic instruction gets surrounded with fence
instruction when needed.
G_FENCE arguments don't have LLT, because of that there is no job for
legalizer and regbankselect. Instruction select G_FENCE for MIPS32.

Differential Revision: https://reviews.llvm.org/D67181

llvm-svn: 371056