[AArch64] Override isLSRCostLess, take number of instructions into account (#84189)

Adds an AArch64-specific version of isLSRCostLess, changing the relative
importance of the various terms from the formulae being evaluated.

This has been split out from my vscale-aware LSR work, see the RFC for
reference:
https://discourse.llvm.org/t/rfc-vscale-aware-loopstrengthreduce/77131