[llvm] [DAGCombine] Use isLegalExtLoad for MatchLoadCombine (PR #140536)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

The load and store legality functions are a mess, technically this probably should also check the legality of the load. Really we need a function that takes the memory type, the extension type, alignment and address space (something like #131424)

https://github.com/llvm/llvm-project/pull/140536