i965: Add negative_equals methods

This method is similar to the existing ::equals methods.  Instead of
testing that two src_regs are equal to each other, it tests that one is
the negation of the other.

v2: Simplify various checks based on suggestions from Matt.  Use
src_reg::type instead of fixed_hw_reg.type in a check.  Also suggested
by Matt.

v3: Rebase on 3 years.  Fix some problems with negative_equals with VF
constants.  Add fs_reg::negative_equals.

v4: Replace the existing default case with BRW_REGISTER_TYPE_UB,
BRW_REGISTER_TYPE_B, and BRW_REGISTER_TYPE_NF.  Suggested by Matt.
Expand the FINISHME comment to better explain why it isn't already
finished.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> [v3]
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_shader.h

@@ -59,6 +59,7 @@ struct backend_reg : private brw_reg
    }
 
    bool equals(const backend_reg &r) const;
+   bool negative_equals(const backend_reg &r) const;
 
    bool is_zero() const;
    bool is_one() const;