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.cpp

@@ -684,6 +684,12 @@ backend_reg::equals(const backend_reg &r) const
    return brw_regs_equal(this, &r) && offset == r.offset;
 }
 
+bool
+backend_reg::negative_equals(const backend_reg &r) const
+{
+   return brw_regs_negative_equal(this, &r) && offset == r.offset;
+}
+
 bool
 backend_reg::is_zero() const
 {