[clang] Fix ignoring converted first arg of elementwise_fma (#154941)

Fixes #154900

Author: tbaederr

clang/lib/Sema/SemaChecking.cpp

@@ -15955,6 +15955,7 @@ bool Sema::BuiltinElementwiseTernaryMath(
       return true;
   }
 
+  TheCall->setArg(0, Args[0]);
   for (int I = 1; I < 3; ++I) {
     if (Args[0]->getType().getCanonicalType() !=
         Args[I]->getType().getCanonicalType()) {

clang/test/Sema/constant-builtins-vector.cpp

@@ -957,3 +957,7 @@ static_assert(fmaDouble1[0] == 5.0);
 static_assert(fmaDouble1[1] == 10.0);
 static_assert(fmaDouble1[2] == 17.0);
 static_assert(fmaDouble1[3] == 26.0);
+
+constexpr float fmaArray[] = {2.0f, 2.0f, 2.0f, 2.0f};
+constexpr float fmaResult = __builtin_elementwise_fma(fmaArray[1], fmaArray[2], fmaArray[3]);
+static_assert(fmaResult == 6.0f, "");