Split PlaceContext::Store into Store & AsmOutput

Split PlaceContext::Store into Store & AsmOutput

Outputs in InlineAsm can be read-write, so splitting it out is useful for things like Store-Store folding, as it cannot be done for a Store-AsmOutput.

This PR is intended to make no changes, just be the mechanical split of the enum.  Future changes can use the split, like a MIR pass I'm working on and perhaps two-phase borrows.