indvars -disable-iv-rewrite: Adds support for eliminating identity

ops.

This is a rewrite of the IV simplification algorithm used by
-disable-iv-rewrite. To avoid perturbing the default mode, I
temporarily split the driver and created SimplifyIVUsersNoRewrite. The
idea is to avoid doing opcode/pattern matching inside
IndVarSimplify. SCEV already does it. We want to optimize with the
full generality of SCEV, but optimize def-use chains top down on-demand rather
than rewriting the entire expression bottom-up. This was easy to do
for operations that SCEV can prove are identity function. So we're now
eliminating bitmasks and zero extends this way.

A result of this rewrite is that indvars -disable-iv-rewrite no longer
requires IVUsers.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133502 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/IndVarSimplify/iv-zext.ll

@@ -1,4 +1,5 @@
 ; RUN: opt < %s -indvars -S | FileCheck %s
+; RUN: opt < %s -indvars -disable-iv-rewrite -S | FileCheck %s
 ; CHECK-NOT: and
 ; CHECK-NOT: zext