Rollup merge of rust-lang#49231 - gnzlbg:fix_vec_fminmax, r=rkruppe

fix vector fmin/fmax non-fast/fast intrinsics NaN handling

This bugs shows up in release mode tests of `stdsimd`: rust-lang/stdarch#391 . The intrinsics are thoroughly tested there for roundoff errors, NaN, and overflow behavior.

The problem was that the non-fast intrinsics where specifying `NoNaNs == true`, which meant that they don't support NaNs. This is incorrect, the non-fast intrinsics should handle NaNs properly.

Also, the "fast" intrinsics where specifying `NoNaNs == false` which meant that they support NaNs and then fast-math, which probably disables this support. This was not intended either.

I've added a comment specifying what the boolean flags do.

src/librustc_trans/builder.rs

@@ -1036,7 +1036,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn vector_reduce_fmin(&self, src: ValueRef) -> ValueRef {
         self.count_insn("vector.reduce.fmin");
         unsafe {
-            let instr = llvm::LLVMRustBuildVectorReduceFMin(self.llbuilder, src, true);
+            let instr = llvm::LLVMRustBuildVectorReduceFMin(self.llbuilder, src, /*NoNaNs:*/ false);
             if instr.is_null() {
                 bug!("LLVMRustBuildVectorReduceFMin is not available in LLVM version < 5.0");
             }
@@ -1046,7 +1046,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn vector_reduce_fmax(&self, src: ValueRef) -> ValueRef {
         self.count_insn("vector.reduce.fmax");
         unsafe {
-            let instr = llvm::LLVMRustBuildVectorReduceFMax(self.llbuilder, src, true);
+            let instr = llvm::LLVMRustBuildVectorReduceFMax(self.llbuilder, src, /*NoNaNs:*/ false);
             if instr.is_null() {
                 bug!("LLVMRustBuildVectorReduceFMax is not available in LLVM version < 5.0");
             }
@@ -1056,7 +1056,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn vector_reduce_fmin_fast(&self, src: ValueRef) -> ValueRef {
         self.count_insn("vector.reduce.fmin_fast");
         unsafe {
-            let instr = llvm::LLVMRustBuildVectorReduceFMin(self.llbuilder, src, false);
+            let instr = llvm::LLVMRustBuildVectorReduceFMin(self.llbuilder, src, /*NoNaNs:*/ true);
             if instr.is_null() {
                 bug!("LLVMRustBuildVectorReduceFMin is not available in LLVM version < 5.0");
             }
@@ -1067,7 +1067,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn vector_reduce_fmax_fast(&self, src: ValueRef) -> ValueRef {
         self.count_insn("vector.reduce.fmax_fast");
         unsafe {
-            let instr = llvm::LLVMRustBuildVectorReduceFMax(self.llbuilder, src, false);
+            let instr = llvm::LLVMRustBuildVectorReduceFMax(self.llbuilder, src, /*NoNaNs:*/ true);
             if instr.is_null() {
                 bug!("LLVMRustBuildVectorReduceFMax is not available in LLVM version < 5.0");
             }