[FS-AFDO] Load pseudo probe profile on MIR

This change enables loading pseudo-probe based profile on MIR. Different from the IR profile loader, callsites are excluded from MIR profile loading since they are not assinged a FS discriminator. Using zero as the discriminator is not accurate and would undo the distribution work done by the IR loader based on pseudo probe distribution factor. We reply on block probes only for FS profile loading.

Some refactoring is done to the IR profile loader so that `getProbeWeight` can be shared by both loaders.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D148584