Merge pull request #3471 from Sonicadvance1/another_3421_bug

ASM: Another sign extend bug in #3421

unittests/32Bit_ASM/FEX_bugs/SignExtendBug.asm

@@ -2,13 +2,16 @@
 {
   "RegData": {
     "RAX": "0x41424344",
-    "RBX": "0x41424344"
+    "RBX": "0x41424344",
+    "RCX": "0x51525354"
   },
   "MemoryRegions": {
+    "0x00fd0000": "4096",
     "0xf0000000": "4096"
   },
   "MemoryData": {
-    "0xf0000000": "0x41424344"
+    "0xf0000000": "0x41424344",
+    "0x00fd0000": "0x51525354"
   },
   "Mode": "32BIT"
 }
@@ -27,4 +30,14 @@ jmp .test
 .test:
 mov ebx, [ebx+ecx]
 
+; Ensures that zext occurs correctly with SIB indexing with second argument not having sign bit set but "index" having sign bit.
+; Originally saw in Metal Gear Rising Revengeance with a `jmp dword [ecx*4+0xfdbf10]` instruction.
+; With ecx = 0xfffffff4 = -12. This is them loading a switch table's branches just before the switch base.
+mov ecx, -12
+
+; Break the block so it can't optimize through.
+jmp .test2
+.test2:
+
+mov ecx, [ecx*4+0x00fd_0030]
 hlt