Fix shader crash when using a varying in separate func before it defined #35406

Job	Run time
📊 Static checks / Code style, file formatting, and docs	33s
🍎 macOS / Editor (target=editor, tests=yes)	4m 11s
🐧 Linux / Editor w/ Mono (target=editor)	4m 30s
🍎 macOS / Template (target=template_release)	3m 21s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	37m 32s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 25s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 5s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	9m 2s
🤖 Android / Editor (target=editor)	4m 10s
🌐 Web / Template w/o threads (target=template_release, threads=no)	6m 19s
🏁 Windows / Editor (target=editor, tests=yes)	8m 10s
🐧 Linux / Template w/ Mono (target=template_release)	4m 17s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	4m 2s
🏁 Windows / Template (target=template_release)	6m 57s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 0s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 53s
🍏 iOS / Template (target=template_release)	2m 10s
🪲 Godot CPP / Build and test Godot CPP	7m 53s
	2h 2m 30s

Provide feedback