fsxc: micro-optimization #1892

Job	Run time
linux-newLTS-github--dotnet-and-msbuild	4m 2s
linux-newLTS-github--dotnet-and-xbuild	3m 50s
linux-newLTS-vanilla--dotnet6	4m 7s
linux-newLTS-vanilla--newmono	3m 23s
linux-newLTS-vanilla--stockmono	2m 46s
linux-oldLTS-github--dotnet-and-msbuild	4m 1s
linux-oldLTS-github--dotnet-and-xbuild	4m 2s
linux-oldLTS-vanilla--newmono	3m 38s
linux-oldLTS-vanilla--stockmono	2m 58s
macOS--mono	3m 0s
macOS--dotnet6-and-mono	3m 48s
windows--dotnet6	4m 52s
windows--legacyFramework	4m 2s
sanity-check	0s
publish	0s
	48m 29s

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