Tweak Invariant IndexOf logic

[MihaZupan]

Follow-up after #108499
Contributes to #108669

Reverts the change on the case-sensitive path, and changes the ignore-case path to create the remainingSource slice only when it's needed. This takes away some overhead when we do find a match, and avoids having to keep the extra state around during the main loop.

Example change:

Method	Toolchain	input	value	comparisonType	Mean	Error	Ratio
IndexOfString	Main	AAAAAAAAAAAA(...)AAAAAAAAAAAA [100]	x	InvariantCultureIgnoreCase	195.51 ns	0.294 ns	1.00
IndexOfString	PR	AAAAAAAAAAAA(...)AAAAAAAAAAAA [100]	x	InvariantCultureIgnoreCase	173.39 ns	0.200 ns	0.89
IndexOfString	Main	ABCDE	c	InvariantCultureIgnoreCase	19.07 ns	0.066 ns	1.00
IndexOfString	PR	ABCDE	c	InvariantCultureIgnoreCase	18.10 ns	0.031 ns	0.95

MihuBot/runtime-utils#710
MihuBot/runtime-utils#711

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[tarekgh]

Thanks @MihaZupan for your help with this issue!

[tarekgh]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/runtime/actions/runs/11262961938

[tarekgh]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/11262967394

[radekdoulik]

Nice! This improved the performance on wasm