[RISCV] Support constant hoisting of immediate store values (#96073)
Previously getIntImmInstCost only calculated the cost of materialising
the argument of a store if it was the address. This means
ConstantHoisting's transformation wouldn't kick in for cases like
storing two values that require multiple instructions to materialise but
where one can be cheaply generated from the other (e.g. by an addition).
Two key changes were needed to avoid regressions when enabling this:
* Allowing constant materialisation cost to be calculated assuming
zeroes are free (as might happen if you had a 2*XLEN constant and one
half is zero).
* Avoiding constant hoisting if we have a misaligned store that's going
to be a legalised to a sequence of narrower stores. I'm seeing cases
where hoisting the constant ends up with worse codegen in that case.
Out of caution and so as not to unexpectedly degrade other existing hoisting logic, FreeZeroes is used only for the new cost calculations for the load instruction. It would likely make sense to revisit this later.
llvm-project
8687f7cd - [RISCV] Support constant hoisting of immediate store values (#96073)
