x86/Intel: don't modify equates' expressions

x86/Intel: don't modify equates' expressions

Equates involving registers were mis-treated when parsing insn operand
expressions: When "pulling out" the register(s), they would have got
converted to O_constant. While other (local) parsing code was able to cope
with this, the generic part of the assembler was misled. A visible bad
effect would be that local absolute symbols would appear in the symbol
table, when really that should be register symbols (which wouldn't be put
in the symbol table at all).

Clone symbols / expressions as necessary before modifying them.