change remap from FxHashSet to IndexVec in StateTransform mir-opt

[SparrowLii]

This PR fixes the FIXME in the StateTransform mir-opt which change the remap field in TransformVisitor from FxHashSet to IndexVec.
This should save a little time for mir-opt, with relatively a little more memory using

[rust-highfive]

r? @estebank

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[wesleywiser]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 1fb5bdc with merge b474b80eaecf39f4c8cd8d09b8e43618c59c5d5a...

[bors]

☀️ Try build successful - checks-actions
Build commit: b474b80eaecf39f4c8cd8d09b8e43618c59c5d5a (b474b80eaecf39f4c8cd8d09b8e43618c59c5d5a)

[rust-timer]

Queued b474b80eaecf39f4c8cd8d09b8e43618c59c5d5a with parent d8a3fc4, future comparison URL.

[rust-timer]

Finished benchmarking commit (b474b80eaecf39f4c8cd8d09b8e43618c59c5d5a): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[SparrowLii]

It doesn't look like this change will improve performance. Thanks for helping with perf testing, I think this PR can be closed.