Reset MMXState to X87 at the start of each block

Ensures that blocks always start with the same state independently of predecessors
which allows independent compilation of blocks.
Starting in the X87 state is better than starting in MMX state because
MMX state is more work to initialize.

Author: pmatos

FEXCore/Source/Interface/Core/OpcodeDispatcher.h

@@ -112,8 +112,16 @@ class OpDispatchBuilder final : public IREmitter {
     // Changes get stored out by CalculateDeferredFlags.
     CachedNZCV = nullptr;
     CFInverted = CFInvertedABI;
+
     FlushRegisterCache();
 
+    // Start block in X87 state.
+    // This is important to ensure that blocks always start with the same state independently of predecessors
+    // which allows independent compilation of blocks.
+    // Starting in the X87 state is better than starting in MMX state because
+    // MMX state is more work to initialize.
+    MMXState = MMXState_X87;
+
     // New block needs to reset segment telemetry.
     SegmentsNeedReadCheck = ~0U;
 
@@ -2341,6 +2349,9 @@ class OpDispatchBuilder final : public IREmitter {
 
   void ChgStateMMX_X87() override {
     LOGMAN_THROW_A_FMT(MMXState == MMXState_MMX, "Expected state to be MMX");
+    // We explicitly initialize to x87 state in StartNewBlock.
+    // So if we ever change this to do something else, we need to
+    // make sure that we consider if we need to explicitly set it there.
     FlushRegisterCache();
     MMXState = MMXState_X87;
   }