[AArch64] Fix resource length computation for STP. (#81749)

On some uArchs, `STP [s|d], [s|d]` first combines the 2 input registers
in a single register using a vector execution unit. IIUC
AArch64StorePairSuppress tries to prevent forming STPs in case the
critical resource are the vector units, in order to prevent adding more
pressure on those units.

The implementation however simply computes the new critical resource
length by adding resource for another STP. If load/store units are the
critical resource, this means we increase that length by one, and
incorrectly prevent forming the STP.

This patch adjusts the resource computation by also removing 2 STRs, as
introducing a STP will remove 2 single stores. This should more
accurately reflect the resource usage after introducing an STP, and does
not prevent forming STPs if load/store units are the critical resources;
in those cases, STP can actually help to reduce resource usage.

PR: https://github.com/llvm/llvm-project/pull/81749