Change the meaning of 'x' in a transformation

The purpose of this series of commits is to improve code generation
for the Clang compiler.

As a first step we want to change the meaning of 'x' in a
transformation such as:

  operation Literal=q => move Literal x | operation x

Currently, a plain 'x' means reg[0] or x(0), which is the first
element in the X register array. That element is distinct from
r(0) which is a variable in process_main(). Therefore, since r(0)
and x(0) are currently distinct it is fine to use x(0) as a
scratch register.

However, in the next commit we will eliminate the separate variable
for storing the contents of X register zero (thus, x(0) and r(0)
will point to the same location in the X register array). Therefore,
we must use another scratch register in transformation. Redefine
a plain 'x' in a transformation to mean x(1023). Also define
SCRATCH_X_REG so that we can refer to the register by name from
C code.

erts/emulator/beam/beam_load.c

@@ -4322,7 +4322,7 @@ gen_has_map_fields(LoaderState* stp, GenOpArg Fail, GenOpArg Src,
     for (i = 0; i < n; i++) {
 	op->a[3+2*i] = Rest[i];
 	op->a[3+2*i+1].type = TAG_x;
-	op->a[3+2*i+1].val = 0;	/* x(0); normally not used */
+	op->a[3+2*i+1].val = SCRATCH_X_REG; /* Ignore result */
     }
     return op;
 }

erts/emulator/utils/beam_makeops

@@ -601,6 +601,7 @@ sub emulator_output {
     print "#define MAX_GENERIC_OPCODE ", $num_file_opcodes-1, "\n";
     print "#define NUM_GENERIC_OPS ", scalar(@gen_opname), "\n";
     print "#define NUM_SPECIFIC_OPS ", scalar(@op_to_name), "\n";
+    print "#define SCRATCH_X_REG 1023\n";
     print "\n";
     print "#ifdef ARCH_64\n";
     print "#  define BEAM_WIDE_MASK 0xFFFFUL\n";
@@ -1341,7 +1342,10 @@ sub tr_parse_op {
     }
 
     # Get an optional value. (In destination.)
+    $type_val = $type eq 'x' ? 1023 : 0;
     if (/^=(.*)/) {
+	error("value not allowed in source: $op")
+	    if $src;
 	$type_val = $1;
 	$_ = '';
     }
@@ -1360,11 +1364,6 @@ sub tr_parse_op {
 	    if $var && $type;
     }
 
-    # Test that source has no values.
-    if ($src) {
-	error("value not allowed in source: $op")
-	    if $type_val;
-    }
     ($var,$type,$type_val,$cond,$cond_val);
 }