[IndVars][NFC] Refactor to make modifications of Changed transparent
[IndVars][NFC] Refactor to make modifications of Changed transparent

IndVarSimplify's design is somewhat odd in the way how it reports that
some transform has made a change. It has a `Changed` field which can
be set from within any function, which makes it hard to track whether or
not it was set properly after a transform was made. It leads to oversights
in setting this flag where needed, see example in PR38855.

This patch removes the `Changed` field, turns it into a local and unifies
the signatures of all relevant transform functions to return boolean value
which designates whether or not this transform has made a change.

Differential Revision: https://reviews.llvm.org/D51850
Reviewed By: skatkov


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@341893 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm