QMetaTypeId: optimize qt_metatype_id()

QMetaTypeId: optimize qt_metatype_id()

The old code resulted in one memory fetch for the test plus another one
for the return value (at the assembler level). The new code reuses the
already-loaded value:

-       movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %edx
-       testl   %edx, %edx
-       je      .L158
        movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %eax
-       ret
+       testl   %eax, %eax
+       je      .L160
+       rep; ret

It also saves one load in the not-yet-initialised case:

-.L158:
+.L160:
        leaq    .LC7(%rip), %rdi
        subq    , %rsp
        .cfi_def_cfa_offset 16
        movq    himBH1, %rsi
        call    _Z17qRegisterMetaTypeI12QDBusMessageEiPKcPT_
        movl    %eax, _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip)
-       movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %eax
        addq    , %rsp
        .cfi_def_cfa_offset 8
        ret

Change-Id: I769950449822f2b1587680e05518be0a4f3120a2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>