When consecutive branch_fp_* instructions use the same operands (e.g.
branch_fp_unordered followed by branch_fp_equal), the 2nd ucomisd/fcmp
is redundant since flags are still valid from the first comparison.
Track the last FP comparison operands in HandlerState and skip the
comparison instruction when it would be identical. This is common in
the double_equality_compare macro which checks for unordered (NaN)
before testing equality.
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