[RISCV] Add basic support for the sifive-7-series short forward branch optimization.

[RISCV] Add basic support for the sifive-7-series short forward branch optimization.

sifive-7-series has macrofusion support to convert a branch over
a single instruction into a conditional instruction. This can be
an improvement if the branch is hard to predict.

This patch adds support for the most basic case, a branch over a
move instruction. This is implemented as a pseudo instruction so
we can hide the control flow until all code motion passes complete.

I've disabled a recent select optimization if this feature is enabled
in the subtarget.

Related gcc patch for the same optimization https://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg211045.html

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D135814