[GC] Remove redundant entiries in stackmap section (and test it this time)

This is a reimplementation of the optimization removed in D75964. The actual spill/fill optimization is handled by D76013, this one just worries about reducing the stackmap section size itself by eliminating redundant entries. As noted in the comments, we could go a lot further here, but avoiding the degenerate invoke case as we did before is probably "enough" in practice.

Differential Revision: https://reviews.llvm.org/D76021