[RegAlloc] Scale the spill weight by target factor (#113675)

Currently, the spill weight is only determined by isDef/isUse and
block frequency. However, for registers with different register
classes, the costs of spilling them are different.

For example, for `LMUL>1` registers (in which, several physical
registers compound a bigger logical register), the costs are larger
than `LMUL=1` case (in which, there is only one physical register).

To solve this problem, a new target hook `getSpillWeightScaleFactor`
is added. Targets can override the default factor (which is `1.0`)
according to the register class.

For RISC-V, the factors are set to the `RegClassWeight` which is
used to track register pressure. The values of `RegClassWeight`
happen to be the number of register units.

I believe all of the targets with compounded registers can benefit
from this change, but only RISC-V is customized in this patch since
it has widely been agreed to do so. The other targets need more
performance data to go further.

Partially fixes #113489.

0 files changed, 0 insertions, 0 deletions