[mir-opt] GVN some more transmute cases

[scottmcm]

We already did Transmute-then-PtrToPtr; this adds the nearly-identical PtrToPtr-then-Transmute.

It also adds transmute(Foo(x)) → transmute(x), when Foo is a single-field transparent type. That's useful for things like NonNull { pointer: p }.as_ptr().

Found these as I was looking at rust-lang/compiler-team#807-related changes.

r? mir-opt

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit fd92746 with merge c6b7165...

[bors]

☀️ Try build successful - checks-actions
Build commit: c6b7165 (c6b716561da31605db7d1abe0c4785449a0311b6)

[rust-timer]

Finished benchmarking commit (c6b7165): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

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-perf -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	1

Max RSS (memory usage)

Results (primary 0.8%, secondary 3.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	6.5%	[4.7%, 7.7%]	3
Regressions ❌ (secondary)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-3.5%	[-6.0%, -1.4%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[-6.0%, 7.7%]	7

Cycles

Results (primary -2.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

Results (primary -0.0%, secondary 0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.3%]	8
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.6%, 0.3%]	11

Bootstrap: 796.222s -> 796.621s (0.05%)
Artifact size: 336.06 MiB -> 336.04 MiB (-0.01%)