improve constant prop in gcd (#41258)

With this PR:

```julia
julia> f(x) = x * (2//3)
f (generic function with 1 method)

julia> @code_typed f(2.3)
CodeInfo(
1 ─ %1 = Base.mul_float(x, 0.6666666666666666)::Float64
└──      return %1
) => Float64
```

It is a bit unfortunate to have to resort to `@pure` here, but I could
not get it to constant fold any other way. I don't think this usage
should be problematic since the method only accepts `BitInteger`s and
only ever calls methods that really shouldn't be redefined.

fixes #32024

base/intfuncs.jl

@@ -47,11 +47,22 @@ function gcd(a::T, b::T) where T<:Integer
     checked_abs(a)
 end
 
-# binary GCD (aka Stein's) algorithm
-# about 1.7x (2.1x) faster for random Int64s (Int128s)
 function gcd(a::T, b::T) where T<:BitInteger
     a == 0 && return checked_abs(b)
     b == 0 && return checked_abs(a)
+    r = _gcd(a, b)
+    signbit(r) && __throw_gcd_overflow(a, b)
+    return r
+end
+@noinline __throw_gcd_overflow(a, b) = throw(OverflowError("gcd($a, $b) overflows"))
+
+# binary GCD (aka Stein's) algorithm
+# about 1.7x (2.1x) faster for random Int64s (Int128s)
+# Unfortunately, we need to manually annotate this as `@pure` to work around #41694. Since
+# this is used in the Rational constructor, constant prop is something we do care about here.
+# This does call generic functions, so it might not be completely sound, but since `_gcd` is
+# restricted to BitIntegers, it is probably fine in practice.
+@pure function _gcd(a::T, b::T) where T<:BitInteger
     za = trailing_zeros(a)
     zb = trailing_zeros(b)
     k = min(za, zb)
@@ -65,11 +76,8 @@ function gcd(a::T, b::T) where T<:BitInteger
         v >>= trailing_zeros(v)
     end
     r = u << k
-    # T(r) would throw InexactError; we want OverflowError instead
-    r > typemax(T) && __throw_gcd_overflow(a, b)
-    r % T
+    return r % T
 end
-@noinline __throw_gcd_overflow(a, b) = throw(OverflowError("gcd($a, $b) overflows"))
 
 """
     lcm(x, y...)

test/intfuncs.jl

@@ -479,3 +479,11 @@ end
 for b in [-100:-2; 2:100;]
     @test Base.ndigits0z(0, b) == 0
 end
+
+@testset "constant prop in gcd" begin
+    ci = code_typed(() -> gcd(14, 21))[][1]
+    @test ci.code == Any[Core.ReturnNode(7)]
+
+    ci = code_typed(() -> 14 // 21)[][1]
+    @test ci.code == Any[Core.ReturnNode(2 // 3)]
+end