release/19.x: [SimpleLoopUnswitch] Fix LCSSA phi node invalidation #1793

Job	Run time
abi-dump-setup	1m 32s
Build and Test / Lit Tests (ubuntu-latest)	57m 51s
Build and Test / Lit Tests (windows-2019)	1h 34m 44s
Build and Test / Lit Tests (macOS-13)	1h 18m 51s
abi-dump (build-baseline)	32m 36s
abi-dump (build-latest)	32m 31s
abi-compare	34s
	4h 58m 39s

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