R600/SI: Custom lower SI_IF and SI_ELSE to avoid machine verifier errors

SI_IF and SI_ELSE are terminators which also produce a value.  For
these instructions ISel always inserts a COPY to move their value
to another basic block.  This COPY ends up between SI_(IF|ELSE)
and the S_BRANCH* instruction at the end of the block.

This breaks MachineBasicBlock::getFirstTerminator() and also the
machine verifier which assumes that terminators are grouped together at
the end of blocks.

To solve this we coalesce the copy away right after ISel to make sure
there are no instructions in between terminators at the end of blocks.

llvm-svn: 207591