sroa: Fix accidentally dropped condition (#50555)

Fixes the issue noted in [1] (#50522) and
adds a test to make sure that it doesn't regress.

[1]
9b73611#r121641452

Author: aviatesk

base/compiler/ssair/passes.jl

@@ -1107,7 +1107,7 @@ function sroa_pass!(ir::IRCode, inlining::Union{Nothing,InliningState}=nothing)
         struct_typ = widenconst(argextype(val, compact))
         struct_argtyp = argument_datatype(struct_typ)
         if struct_argtyp === nothing
-            if isa(struct_typ, Union)
+            if isa(struct_typ, Union) && is_isdefined
                 lift_comparison!(isdefined, compact, idx, stmt, lifting_cache, 𝕃ₒ)
             end
             continue

test/compiler/irpasses.jl

@@ -1421,3 +1421,20 @@ let src = code_typed1(sroaunswitchunionstruct1, Tuple{Bool, Int, Float64})
 end
 @test sroaunswitchunionstruct2(true, 1, 1.0) === 1
 @test sroaunswitchunionstruct2(false, 1, 1.0) === 1.0
+
+# Test SROA of union into getfield
+struct SingleFieldStruct1
+    x::Int
+end
+struct SingleFieldStruct2
+    x::Int
+end
+function foo(b, x)
+    if b
+        f = SingleFieldStruct1(x)
+    else
+        f = SingleFieldStruct2(x)
+    end
+    getfield(f, :x) + 1
+end
+@test foo(true, 1) == 2