Fix a comment about SILCombine "passes" in Passes.def

Author: eeckstein

include/swift/SILOptimizer/PassManager/Passes.def

@@ -66,12 +66,13 @@
 #endif
 
 /// SWIFT_SILCOMBINE_PASS(Inst)
-///   Similar to SWIFT_FUNCTION_PASS, but defines an instruction pass which is
-///   implemented in swift and is run by the SILCombiner.
+///   Similar to SWIFT_FUNCTION_PASS, but defines an instruction simplification
+///   which is implemented in swift and is run by the SILCombiner.
 ///   The \p Inst argument specifies the instruction class.
 ///
 ///   No further code is need on the C++ side. On the swift side an instruction
-///   pass must be registered for the instruction class with 'registerPass()'.
+///   simplification must be registered for the instruction class with
+///   `registerForSILCombine`.
 ///
 #ifndef SWIFT_SILCOMBINE_PASS
 #define SWIFT_SILCOMBINE_PASS(Inst)