Fix multiple ubsan warnings in i386-dis.c

Commit 39fb3698 "opcodes: Make i386-dis.c thread-safe" introduced
a number of casts to bfd_signed_vma that cause undefined behaviour
with a 32-bit libbfd.  Revert those changes.

	* i386-dis.c (OP_E_memory): Do not cast disp to bfd_signed_vma
	for negation.
	(get32, get32s): Don't use bfd_signed_vma here.