LoopIdiom: Use utility functions.
LoopIdiom: Use utility functions.

The only difference between deleteIfDeadInstruction and
RecursivelyDeleteTriviallyDeadInstructions is that the former also
manually invalidates SCEV. That's unnecessary because SCEV automatically
gets informed when an instruction is deleted via a ValueHandle. NFC.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@228508 91177308-0d34-0410-b5e6-96231b3b80d8

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