radeonsi: Fix typo: "llvm.fs.interp" => "llvm.SI.fs.interp"
radeonsi: Fix typo: "llvm.fs.interp" => "llvm.SI.fs.interp"

Fixes lots of pixel shaders failing to compile with LLVM 3.9 or older.

Trivial.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99013#c4

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler