Revert "[BasicAA] Handle two unknown sizes for GEPs"

Temporarily revert commit 8b1c4e310c2f9686cad925ad81d8e2be10a1ef3c.

After 8b1c4e310c2f the compile-time for `MultiSource/Benchmarks/MiBench/consumer-lame`
dramatically increases with -O3 & LTO, causing issues for builders with
that configuration.

I filed PR48553 with a smallish reproducer that shows a 10-100x compile
time increase.

1 files changed, 4 insertions, 16 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 3e22ce8..caa0635 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1098,22 +1098,6 @@ AliasResult BasicAAResult::aliasGEP(
     const GEPOperator *GEP1, LocationSize V1Size, const AAMDNodes &V1AAInfo,
     const Value *V2, LocationSize V2Size, const AAMDNodes &V2AAInfo,
     const Value *UnderlyingV1, const Value *UnderlyingV2, AAQueryInfo &AAQI) {
-  // If both accesses are unknown size, we can only check whether the
-  // underlying objects are different.
-  if (!V1Size.hasValue() && !V2Size.hasValue()) {
-    // If the other operand is a phi/select, let phi/select handling perform
-    // this check. Otherwise the same recursive walk is done twice.
-    if (!isa<PHINode>(V2) && !isa<SelectInst>(V2)) {
-      AliasResult BaseAlias =
-          aliasCheck(UnderlyingV1, LocationSize::beforeOrAfterPointer(),
-                     AAMDNodes(), UnderlyingV2,
-                     LocationSize::beforeOrAfterPointer(), AAMDNodes(), AAQI);
-      if (BaseAlias == NoAlias)
-        return NoAlias;
-    }
-    return MayAlias;
-  }
-
   DecomposedGEP DecompGEP1 = DecomposeGEPExpression(GEP1, DL, &AC, DT);
   DecomposedGEP DecompGEP2 = DecomposeGEPExpression(V2, DL, &AC, DT);
 
@@ -1172,6 +1156,10 @@ AliasResult BasicAAResult::aliasGEP(
     // instruction.  If one pointer is a GEP with a non-zero index of the other
     // pointer, we know they cannot alias.
 
+    // If both accesses are unknown size, we can't do anything useful here.
+    if (!V1Size.hasValue() && !V2Size.hasValue())
+      return MayAlias;
+
     AliasResult R = aliasCheck(
         UnderlyingV1, LocationSize::beforeOrAfterPointer(), AAMDNodes(),
         V2, V2Size, V2AAInfo, AAQI, nullptr, UnderlyingV2);