i965/fs: Fix return value of fs_inst::regs_read() for BAD_FILE.

i965/fs: Fix return value of fs_inst::regs_read() for BAD_FILE.

Typically BAD_FILE sources are used to mark a source as not present
what implies that no registers are read.  This will become much more
frequent with logical send opcodes which have a large number of
sources, many of them optionally used and marked as BAD_FILE when they
aren't applicable.  It will prove to be useful to be able to rely on
the value of regs_read() regardless of whether a source is present or
not.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>