gas: scfi: make gen_scfi_ops more readable

Replace the scattered and repeated uses of verbose expressions with
variables:
  ginsn_get_src_reg (src1)  -> src1_reg
  ginsn_get_src_type (src1) -> src1_type
etc.

This hopefully makes the logic more readable.  While at it, make some of
the checks more precise:
  - When getting imm value, ensure the src type is GINSN_SRC_IMM,
  - When getting reg, ensure the src type is checked too (GINSN_SRC_REG
    or GINSN_SRC_INDIRECT as appropriate).

ChangeLog:

        * gas/scfi.c (gen_scfi_ops): Add new local vars and reuse them.
	Make some conditionals more precise.