Revert r312724 ("[ARM] Remove redundant vcvt patterns.").

It leads to some improvements, but also a regression for the simple
case, so it's not clearly a good idea.

test/CodeGen/ARM/vcvt.ll now has test coverage to show the difference.

Ultimately, the right solution is probably to custom-lower fp-to-int
conversions, to something like ARMISD::VCVT_F32_S32 plus a bitcast.
It's hard to do the right thing when the implicit bitcast isn't visible
to DAG transforms.

llvm-svn: 314169