move fast reject test out of `SelectionContext::match_impl`.

move fast reject test out of `SelectionContext::match_impl`.

`match_impl` has two call sites. For one of them (within `rematch_impl`)
the fast reject test isn't necessary, because any rejection would
represent a compiler bug.

This commit moves the fast reject test to the other `match_impl` call
site, in `assemble_candidates_from_impls`. This lets us move the fast
reject test outside the `probe` call in that function. This avoids the
taking of useless snapshots when the fast reject test succeeds, which
gives a performance win when compiling the `bitmaps` and `nalgebra`
crates.

Co-authored-by: name <n.nethercote@gmail.com>