thumb: Extend the regexp for handling negative register indexing

Now the same regexp that previously handled cases such as
"ldr r1, [r2, -r3]" also can handle the first operand being omitted
as in "pld [r2, -r3]".

This fixes building vp9_convolve8*neon.asm in thumb mode (and thus,
for Windows Phone as well).

Change-Id: I20c1c3f2bfb2587fb5fa523b863972a7fe30d8ff