inference: fix #41450, replace constant-folded muladd with fma

So that we can get more accurate results when performance doesn't matter

base/compiler/tfuncs.jl

@@ -1522,6 +1522,11 @@ function builtin_tfunction(interp::AbstractInterpreter, @nospecialize(f), argtyp
     if isa(f, IntrinsicFunction)
         if is_pure_intrinsic_infer(f) && _all(@nospecialize(a) -> isa(a, Const), argtypes)
             argvals = anymap(a::Const -> a.val, argtypes)
+            # https://github.com/JuliaLang/julia/issues/41450
+            # enforce no rounding for better accuracy
+            if f === muladd_float
+                f = fma_float
+            end
             try
                 return Const(f(argvals...))
             catch

test/compiler/inference.jl

@@ -3395,3 +3395,9 @@ end
         x.x
     end) == Any[Int]
 end
+
+# https://github.com/JuliaLang/julia/issues/41450
+@test (@eval Module() begin
+    foo(x=1.0) = muladd(1 + eps(x), 1 - eps(x), -1)
+    foo() == foo(1.0)
+end)

test/mpfr.jl

@@ -606,8 +606,7 @@ end
         @test log(x) == log(42)
         @test isinf(log(BigFloat(0)))
         @test_throws DomainError log(BigFloat(-1))
-        # issue #41450
-        @test_skip log2(x) == log2(42)
+        @test log2(x) == log2(42)
         @test isinf(log2(BigFloat(0)))
         @test_throws DomainError log2(BigFloat(-1))
         @test log10(x) == log10(42)