[RISCV] Simplify BP initialisation

[StephenFan]

Summary

We can re-use copyPhysReg rather than writing a specialised copy.

[craig.topper]

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

508

While they aren't necessary, leave the curly braces since the comment makes more than one line below the if.

[StephenFan]

Address craig.topper's comment.

[luismarques]

This is certainly cleaner, in the sense of the code operating at a higher level of abstraction. I wonder if it's worth the indirection cost.
I'll let others bikeshed this :-)

[jrtc27]

In D95227#2521189, @luismarques wrote:

This is certainly cleaner, in the sense of the code operating at a higher level of abstraction. I wonder if it's worth the indirection cost.
I'll let others bikeshed this :-)

Fewer places we have to teach to use a cmove cbp, csp dowstream in CHERI-LLVM :) (although we've already paid that cost, but still, diff reducing is good)

[asb]

LGTM.