Increased TInstr field sizes: allow long jumps and 65535 VM registers (#12777)

* Increased regBx size from 16 to 24 bits to increase jump range in the VM
from 32K to 8M instructions. Fixes #12727

* Increased VM TInst register field sizes to 16 bits to allow up to 65535 VM registers per proc

* Added test case for >255 VM registers

compiler/vmdef.nim

@@ -12,14 +12,14 @@
 
 import ast, idents, options, modulegraphs, lineinfos
 
-type TInstrType* = uint32
+type TInstrType* = uint64
 
 const
   regOBits = 8 # Opcode
-  regABits = 8
-  regBBits = 8
-  regCBits = 8
-  regBxBits = 16
+  regABits = 16
+  regBBits = 16
+  regCBits = 16
+  regBxBits = 24
 
   byteExcess* = 128 # we use excess-K for immediates