[RISCV] Expand mul to shNadd x, (slli x, c) in DAGCombine (#88524)

This expansion is directly inspired by the analogous code in the x86
backend for LEA. shXadd and (this sub-case of) LEA are largely
equivalent.

This is an alternative to
https://github.com/llvm/llvm-project/pull/87105.

This expansion is also supported via the decomposeMulByConstant
callback, but restricted because of interactions with other combines
since that code runs before legalization. As discussed in the other
review, my original plan had been to support post legalization expansion
through the same interface, but that ended up being more complicated
than seems justified.

Instead, lets go ahead and do the general expansion post-legalize. Other
targets use the combine approach, and matching that structure makes it
easier for us to adapt ideas from other targets to RISCV.