[llvm] [RISCV] Support constant hoisting of immediate store values (PR #96073)

[Alex Bradbury via llvm-commits]

asb wrote:

After some more testing I wasn't happy with the risk that the FreeZeroes change might negatively impact hoisting on other instructions unexpectedly (as it did for e.g. test7 in immediates.ll), so I've made a minor modification so that codepath is used only for the new load costing logic I added.

https://github.com/llvm/llvm-project/pull/96073