Use MEM_EXPR only if MEM_P is true

Use MEM_EXPR only if MEM_P is true

On s390x, for input:

(call_insn/u 7 6 11 2 (parallel [
            (set (reg:SI 2 %r2)
                (call (subreg:QI (symbol_ref:SI ("__tls_get_offset") [flags 0x1]) 3)
                    (const_int 0 [0])))
            (clobber (reg:SI 14 %r14))
            (use (unspec:SI [
                        (const_int 0 [0])
                    ] UNSPEC_TLSLDM))
        ]) "/tmp/foo.c":12:26 2602 {*brasl_tls}
     (expr_list:REG_EH_REGION (const_int -2147483648 [0xffffffff80000000])
        (nil))
    (expr_list (use (reg:SI 2 %r2))
        (expr_list (use (reg:SI 12 %r12))
            (nil))))

after r16-1041-g2da641d0170090, get_call_rtx_from returns:

(call (subreg:QI (symbol_ref:SI ("__tls_get_offset") [flags 0x1]) 3)
    (const_int 0 [0]))

and we got

Program received signal SIGSEGV, Segmentation fault.
0x000000000131174f in prepare_call_arguments (
    bb=<basic_block 0x7fffe99dfba0 (2)>, insn=0x7fffe980cc60)
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:6277
6277		fndecl = MEM_EXPR (XEXP (call, 0));
(gdb) bt
    bb=<basic_block 0x7fffe99dfba0 (2)>, insn=0x7fffe980cc60)
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:6277
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:10297
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:10526
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:10579
    at /export/gnu/import/git/sources/gcc/gcc/var-tracking.cc:10616

Update prepare_call_arguments to check MEM_P before using MEM_EXPR.

gcc/

	PR debug/120525
	* var-tracking.cc (prepare_call_arguments): Use MEM_EXPR only
	if MEM_P is true.

gcc/testsuite/

	PR debug/120525
	* gcc.dg/pr120525.c: New test.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>