Use StartsWith literal optimization for OrdinalIgnoreCase in RegexCompiler / source generator

[stephentoub]

#66095 is optimizing MemoryExtensions.StartsWith(span, "literal", OrdinalIgnoreCase) by auto-vectorizing cases where the literal string is all ASCII. In Regex, we can take advantage of this in the compiler and source generator by recognizing sequences of appropriate sets, e.g. "[Aa][Bb][Cc]", which our IgnoreCase option now produces (and will produce more robustly with #61048). Today we'll output individual checks for each set, e.g. (ch | 0x20) == 'a', but we can start outputting StartsWith calls with the corresponding multi-character literal string rather than performing individual comparisons against each char.

cc: @EgorBo, @joperezr

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Issue Details

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#66095 is optimizing MemoryExtensions.StartsWith(span, "literal", OrdinalIgnoreCase) by auto-vectorizing cases where the literal string is all ASCII. In Regex, we can take advantage of this in the compiler and source generator by recognizing sequences of appropriate sets, e.g. "[Aa][Bb][Cc]", which our IgnoreCase option now produces (and will produce more robustly with #61048). Today we'll output individual checks for each set, e.g. (ch | 0x20) == 'a', but we can start outputting StartsWith calls with the corresponding multi-character literal string rather than performing individual comparisons against each char.

cc: @EgorBo, @joperezr

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Text.RegularExpressions, tenet-performance
Milestone:	7.0.0