This implements two hooks that attempt to avoid control flow for RISC-V. RISC-V
will lower SELECTs into control flow, which is not a great idea.
The hook hasMultipleConditionRegisters() turns off the following
select(C0|C1, x, y) <=> select(C0, x, select(C1, x, y)) select(C0&C1, x, y) <=> select(C0, select(C1, x, y), y)
The second hook setJumpIsExpensive controls a flag that has a similar purpose
and is used in CodeGenPrepare and the SelectionDAGBuilder.
Both of these have the effect of ensuring more logic is done before fewer jumps.
Note: with the B extension, we may be able to lower select into a conditional
move instruction, so at some point these hooks will need to be guarded based on