LoongArch: Avoid RTL flag check failure in loongarch_classify_symbol

LoongArch: Avoid RTL flag check failure in loongarch_classify_symbol

SYMBOL_REF_TLS_MODEL invokes SYMBOL_REF_FLAGS, and SYMBOL_REF_FLAGS
invokes RTL_FLAG_CHECK1 and aborts when RTL code is not SYMBOL_REF.

r13-1833 removed "gcc_assert (SYMBOL_REF_P (x))" before invoking
"SYMBOL_REF_TLS_MODEL (x)", indicating that it's now possible that "x"
is not a SYMBOL_REF.  So we need to check if "x" is SYMBOL_REF first.

This fixes a test failure happening with r13-2173 with RTL flag
checking enabled:

    pr106096.C:26:1: internal compiler error: RTL flag check:
    SYMBOL_REF_FLAGS used with unexpected rtx code 'const' in
    loongarch_classify_symbol

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_classify_symbol):
	Return early if the rtx is not SYMBOL_REF.