[SimplifyCFG] propagate fast-math-flags (FMF) from phi to select

This is another step towards having FMF apply only to FP values
rather than those + fcmp. See PR38086 for one of the original
discussions/motivations:
https://bugs.llvm.org/show_bug.cgi?id=38086

And the test here is derived from PR39535:
https://bugs.llvm.org/show_bug.cgi?id=39535

Currently, we lose FMF when converting any phi to select in
SimplifyCFG. There are a small number of similar changes needed
to correct within SimplifyCFG, so it should be quick to patch
this pass up.

FMF was extended to select and phi with:
D61917
D67564

Differential Revision: https://reviews.llvm.org/D70208

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1404,10 +1404,16 @@ HoistTerminator:
       // These values do not agree.  Insert a select instruction before NT
       // that determines the right value.
       SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
-      if (!SI)
+      if (!SI) {
+        // Propagate fast-math-flags from phi node to its replacement select.
+        IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
+        if (isa<FPMathOperator>(PN))
+          Builder.setFastMathFlags(PN.getFastMathFlags());
+
         SI = cast<SelectInst>(
             Builder.CreateSelect(BI->getCondition(), BB1V, BB2V,
                                  BB1V->getName() + "." + BB2V->getName(), BI));
+      }
 
       // Make the PHI node use the select for all incoming values for BB1/BB2
       for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)

llvm/test/Transforms/SimplifyCFG/HoistCode.ll

@@ -19,7 +19,7 @@ define float @PR39535min(float %x) {
 ; CHECK-LABEL: @PR39535min(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = fcmp une float [[X:%.*]], 0.000000e+00
-; CHECK-NEXT:    [[DOTX:%.*]] = select i1 [[TOBOOL]], float 0.000000e+00, float [[X]]
+; CHECK-NEXT:    [[DOTX:%.*]] = select fast i1 [[TOBOOL]], float 0.000000e+00, float [[X]]
 ; CHECK-NEXT:    ret float [[DOTX]]
 ;
 entry: