[IndVars] Fix PR25576
`LCSSASafePhiForRAUW` as computed was incorrect -- in cases like
these (this exact example does not actually trigger the bug):
define i32 @f(i32 %n, i1* %c) {
entry:
br label %outer.loop
outer.loop:
br label %inner.loop
inner.loop:
%iv = phi i32 [ 0, %outer.loop ], [ %iv.inc, %inner.loop ]
%iv.inc = add nuw nsw i32 %iv, 1
%tc = udiv i32 %n, 13
%be.cond = icmp ult i32 %iv, %tc
br i1 %be.cond, label %inner.loop, label %inner.exit
inner.exit:
%iv.lcssa = phi i32 [ %iv, %inner.loop ]
%outer.be.cond = load volatile i1, i1* %c
br i1 %outer.be.cond, label %outer.loop, label %leave
leave:
%iv.lcssa.lcssa = phi i32 [ %iv.lcssa, %inner.exit ]
ret i32 %iv.lcssa.lcssa
}
`LCSSASafePhiForRAUW` is true for `%iv.lcssa` when re-rewriting the exit
value of `%iv` for `%inner.loop` to `%tc` (this can happen due to
`SCEVExpander::findExistingExpansion`), but the RAUW breaks LCSSA.
To fix this, instead of computing `SafePhi` with special logic, decide
the safety of RAUW directly via `replacementPreservesLCSSAForm`.
llvm-svn: 258016
