Auto merge of #120718 - saethlin:reasonable-fast-math, r=nnethercote

Add "algebraic" fast-math intrinsics, based on fast-math ops that cannot return poison

Setting all of LLVM's fast-math flags makes our fast-math intrinsics very dangerous, because some inputs are UB. This set of flags permits common algebraic transformations, but according to the [LangRef](https://llvm.org/docs/LangRef.html#fastmath), only the flags `nnan` (no nans) and `ninf` (no infs) can produce poison.

And this uses the algebraic float ops to fix https://github.com/rust-lang/rust/issues/120720

cc `@orlp`

compiler/rustc_span/src/symbol.rs

@@ -764,8 +764,10 @@ symbols! {
         f64_nan,
         fabsf32,
         fabsf64,
+        fadd_algebraic,
         fadd_fast,
         fake_variadic,
+        fdiv_algebraic,
         fdiv_fast,
         feature,
         fence,
@@ -785,6 +787,7 @@ symbols! {
         fmaf32,
         fmaf64,
         fmt,
+        fmul_algebraic,
         fmul_fast,
         fn_align,
         fn_delegation,
@@ -810,6 +813,7 @@ symbols! {
         format_unsafe_arg,
         freeze,
         freg,
+        frem_algebraic,
         frem_fast,
         from,
         from_desugaring,
@@ -823,6 +827,7 @@ symbols! {
         from_usize,
         from_yeet,
         fs_create_dir,
+        fsub_algebraic,
         fsub_fast,
         fundamental,
         future,