[RISCV] Allow conversion of CC logic to bitwise logic

Indicates in the TargetLowering interface that conversions from CC logic to
bitwise logic are allowed. Adds tests that show the benefit when optimization
opportunities are detected. Also adds tests that show that when the optimization
is not applied correct code is generated (but opportunities for other
optimizations remain).

Differential Revision: https://reviews.llvm.org/D59596
Patch by Luís Marques.

llvm-svn: 356740