Don't set fast(-math) for certain simd ops

`fast-math` implies things like functions not being able to accept as an
argument or return as a result, say, `inf` which made these functions
confusingly named or behaving incorrectly, depending on how you
interpret it. Since the time when these intrinsics have been implemented
the intrinsics user's (stdsimd) approach has changed significantly and
so now it is required that these intrinsics operate normally rather than
in "whatever" way.

Fixes #84268

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -349,8 +349,10 @@ extern "C" void LLVMRustRemoveFunctionAttributes(LLVMValueRef Fn,
   F->setAttributes(PALNew);
 }
 
-// enable fpmath flag UnsafeAlgebra
-extern "C" void LLVMRustSetHasUnsafeAlgebra(LLVMValueRef V) {
+// Enable a fast-math flag
+//
+// https://llvm.org/docs/LangRef.html#fast-math-flags
+extern "C" void LLVMRustSetFastMath(LLVMValueRef V) {
   if (auto I = dyn_cast<Instruction>(unwrap<Value>(V))) {
     I->setFast(true);
   }