[Mips][FastISel] Fix selectBranch on icmp i1

The r337288 tried to fix result of icmp i1 when its input is not sanitized
by falling back to DagISel. While it now produces the correct result for
bit 0, the other bits can still hold arbitrary value which is not supported
by MipsFastISel branch lowering. This patch fixes the issue by falling back
to DagISel in this case.

Patch by Dragan Mladjenovic.

Differential Revision: https://reviews.llvm.org/D52045

llvm-svn: 342884