Auto merge of #74748 - simonvandel:simplify-discriminant-arm, r=wesleywiser

MIR-OPT: Make SimplifyBranchSame able to remove identity match with fieldless variant

Modifies SimplifyBranchSame so that it can see that the statements can be considered equal in the following example
`_0 = _1` and `discriminant(_0) = discriminant(0)` are considered equal if 0 is a fieldless variant of an enum.