Disable pathologically expensive SimplifySelectOps optimization

`SimplifySelectOps` is a late optimization in LLVM that attempts to
translate `select(C, load(A), load(B))` into `load(select(C, A, B))`.
However, in order for it to do this optimization, it needs to check that
`C` does not depend on the result of `load(A)` or `load(B)`.
Unfortunately (unlikely Julia and LLVM at the IR level), LLVM does not
have a topological order of statements computed at this stage of the
compiler, so LLVM needs to iterate through all statements in the
function in order to perform this legality check. For large functions,
this is extremely expensive, accounting for the majority of all
compilation time for such functions. On the other hand, the optimization
itself is minor, allowing at most the elision of one additional load
(and doesn't fire particularly often, because the middle end can perform
similar optimizations). Until there is a proper solution in LLVM, simply
disable this optimizations, making LLVM several orders of magnitude
faster on real world benchmarks.

X-ref: llvm#60132

Author: staticfloat

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -23772,6 +23772,7 @@ bool DAGCombiner::SimplifySelectOps(SDNode *TheSelect, SDValue LHS,
         !TLI.isOperationLegalOrCustom(TheSelect->getOpcode(),
                                       LLD->getBasePtr().getValueType()))
       return false;
+    return false;
 
     // The loads must not depend on one another.
     if (LLD->isPredecessorOf(RLD) || RLD->isPredecessorOf(LLD))