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

tests/vm/tmanyregs.nim

@@ -0,0 +1,16 @@
+import macros
+
+# Generate a proc with more then 255 registers. Should not generate an error at
+# compile time
+
+static:
+  macro mkFoo() =
+    let ss = newStmtList()
+    for i in 1..256:
+      ss.add parseStmt "var x" & $i & " = " & $i
+      ss.add parseStmt "inc x" & $i
+    quote do:
+      proc foo() =
+        `ss`
+  mkFoo()
+  foo()