Fix PR 26000, logical bitwise error / prologue analyzer

Fix PR 26000, logical bitwise error / prologue analyzer

This fixes an instruction mask typo. We should be matching only
ldrd (immediate) and not any other of its variants. As is, it never matches
anything.

With the patch, the instruction mask also allows matching of ldrd (literal),
but the check for SP discards this particular instruction pattern, as it has
a hardcoded PC register.

gdb/ChangeLog:

2020-05-27  Luis Machado  <luis.machado@linaro.org>

	PR tdep/26000
	* arm-tdep.c (thumb_analyze_prologue): Fix instruction matching
	for ldrd (immediate).