radeonsi: fix a regression in si_eliminate_const_output
radeonsi: fix a regression in si_eliminate_const_output

A constant value of float type is not necessarily a ConstantFP: it could also
be a constant expression that for some reason hasn't been folded.

This fixes a regression in GL45-CTS.arrays_of_arrays_gl.InteractionFunctionCalls2
that was introduced by commit 3ec9975555d1cc5365413ad9062f412904f944a3.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler