cmd/compile: use better splitting condition for string binary search

Currently we use a full cmpstring to do the comparison for each
split in the binary search for a string switch.

Instead, split by comparing a single byte of the input string with a
constant. That will give us a much faster split (although it might be
not quite as good a split).

Fixes #53333

R=go1.20

Change-Id: I28c7209342314f367071e4aa1f2beb6ec9ff7123
Reviewed-on: https://go-review.googlesource.com/c/go/+/414894
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

test/codegen/switch.go

@@ -72,3 +72,30 @@ func length(x string) int {
 		return len(x)
 	}
 }
+
+// Use single-byte ordered comparisons for binary searching strings.
+// See issue 53333.
+func mimetype(ext string) string {
+	// amd64: `CMPB\s1\(.*\), \$104$`,-`cmpstring`
+	// arm64: `MOVB\s1\(R.*\), R.*$`, `CMPW\s\$104, R.*$`, -`cmpstring`
+	switch ext {
+	// amd64: `CMPL\s\(.*\), \$1836345390$`
+	// arm64: `CMPW\s\$1836345390, R.*$`
+	case ".htm":
+		return "A"
+	// amd64: `CMPL\s\(.*\), \$1953457454$`
+	// arm64: `CMPW\s\$1953457454, R.*$`
+	case ".eot":
+		return "B"
+	// amd64: `CMPL\s\(.*\), \$1735815982$`
+	// arm64: `CMPW\s\$1735815982, R.*$`
+	case ".svg":
+		return "C"
+	// amd64: `CMPL\s\(.*\), \$1718907950$`
+	// arm64: `CMPW\s\$1718907950, R.*$`
+	case ".ttf":
+		return "D"
+	default:
+		return ""
+	}
+}