spirv: Relax the validation conditions of OpSelect

spirv: Relax the validation conditions of OpSelect

The Talos Principle contains shaders with an OpSelect between two
vectors where the condition is a scalar boolean.  This is technically
against the spec bout nir_builder gracefully handles it by splatting
out the condition to all the channels.  So long as the condition is a
boolean, just emit a warning instead of failing.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104246