llvmpipe: fix texcoord analysis regression

llvmpipe: fix texcoord analysis regression

The texcoord component can be specifed by the src registers swizzle
or the variable's location_frac field.

Signed-off-by: Brian Paul <brianp@vmware.com>
Fixes: 90d011de8ee ("llvmpipe: fix texcoord analysis in llvmpipe_nir_fn_is_linear_compat()")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6889
Reviewed-by: Emma Anholt <emma@anholt.net>
Tested-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17743>