[RISCV] Explicitly select second operand of branch condition to X0.

At least based on the lit tests, the coalescer sometimes fails to
propagate the copy from X0 into the branch instruction. This patch
does it manually during isel. The majority of the changes are from
the select patterns.

Some of the changes are just register allocation changes. Only
the Select change affects the whether a b*z instruction is generated
in the tests. I changed the branch pattern for consistency.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D130809