JIT: ensure fgFirstBB has appropriate flags (dotnet#40038)

This fixes an issue where release jits might sometimes generate bad GC info.
Keeping it minimal for now so we can consider servicing preview 8.

See dotnet#39023 for details.

src/coreclr/src/jit/flowgraph.cpp

@@ -175,6 +175,8 @@ void Compiler::fgInit()
 #ifdef FEATURE_SIMD
     fgPreviousCandidateSIMDFieldAsgStmt = nullptr;
 #endif
+
+    fgHasSwitch = false;
 }
 
 bool Compiler::fgHaveProfileData()
@@ -476,7 +478,8 @@ void Compiler::fgEnsureFirstBBisScratch()
 
     noway_assert(fgLastBB != nullptr);
 
-    block->bbFlags |= (BBF_INTERNAL | BBF_IMPORTED);
+    // Set the expected flags
+    block->bbFlags |= (BBF_INTERNAL | BBF_IMPORTED | BBF_JMP_TARGET | BBF_HAS_LABEL);
 
     // This new first BB has an implicit ref, and no others.
     block->bbRefs = 1;