gas: scfi: bugfixes for SCFI state propagation

gas: scfi: bugfixes for SCFI state propagation

There are two state propagation functions in SCFI machinery - forward
and backward flow.  The patch addresses two issues:
  - In forward_flow_scfi_state (), the state being compared in forward flow
    must be that at the exit of a prev bb and that at the entry of the
    next bb.  The variable holding the state to be compared was
    previously (erroneously) stale.
  - In cmp_scfi_state (), the assumption that two different control
    flows, leading to the same basic block, cannot have a mismatched
    notion of CFA base register, is not true.  Remove the assertion and
    instead return err if mismatch.

Fixing these issues helps correctly synthesize CFI, when previously
SCFI was erroring out for an otherwise valid input asm.

gas/
	* scfi.c (cmp_scfi_state): Remove assertion and return mismatch
	in return value as applicable.
	(forward_flow_scfi_state): Update state object to be the same as
	the exit state of the prev bb before comparing.

gas/testsuite/
	* gas/scfi/x86_64/scfi-x86-64.exp: Add new test.
	* gas/scfi/x86_64/scfi-cfg-5.d: New test.
	* gas/scfi/x86_64/scfi-cfg-5.l: New test.
	* gas/scfi/x86_64/scfi-cfg-5.s: New test.