[Mips][FastISel] Do not duplicate condition while lowering branches

This change fixes the issue that arises when we duplicate condition from
the predecessor block. If the condition's arguments are not considered alive
across the blocks, fast regalloc gets confused and starts generating reloads
from the slots that have never been spilled to. This change also leads to
smaller code given that, unlike on architectures with condition codes, on
Mips we can branch directly on register value, thus we gain nothing by
duplication.

Patch by Dragan Mladjenovic.

Differential Revision: https://reviews.llvm.org/D48642

llvm-svn: 336084