Optimizer: re-implement the RedundantLoadElimination pass in Swift

The new implementation has several benefits compared to the old C++ implementation:

* It is significantly simpler. It optimizes each load separately instead of all at once with bit-field based dataflow.
* It's using alias analysis more accurately which enables more loads to be optimized
* It avoids inserting additional copies in OSSA

The algorithm is a data flow analysis which starts at the original load and searches for preceding stores or loads by following the control flow in backward direction.
The preceding stores and loads provide the "available values" with which the original load can be replaced.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CMakeLists.txt

@@ -17,6 +17,7 @@ swift_compiler_sources(Optimizer
   ObjCBridgingOptimization.swift
   MergeCondFails.swift
   NamedReturnValueOptimization.swift
+  RedundantLoadElimination.swift
   ReleaseDevirtualizer.swift
   SimplificationPasses.swift
   StackPromotion.swift