Microoptimize type-tests on x86oids.

Microoptimize type-tests on x86oids.

On x86-64 in %test-lowtag instead of doing:

MOV EAX, ECX
AND AL, 15
CMP AL, 15
do
LEA EAX, [RCX-15]
TEST AL, 15

Which allows to save one byte.

On x86 this optimization is already applied, but since LEA loads a
32-bit integer, EAX can be later used as an already untagged pointer
in %test-headers: MOV EAX, [ECX-7] => MOV EAX, [EAX], which takes one
byte less to encode.