instr_is_encoding_possible fails on unreachable pc-relative operands but takes no target

While adding a test for #5302 (closed) on arm, I hit this:

ASSERT FAILURE: /home/derek/dr/git/src/suite/tests/api/ir_arm.c:80: instr_is_encoding_possible(instr)

This is for the instruction in test_pcrel() in ir_arm.c.

The problem is that instr_is_encoding_possible() does not take in a target location for encoding, so it can't know whether pc-relative operands will reach. It has to relax those reachability checks.