RISC-V: Bugfix for legitimize address PR/111634

RISC-V: Bugfix for legitimize address PR/111634

Given we have RTL as below.

(plus:DI (mult:DI (reg:DI 138 [ g.4_6 ])
                  (const_int 8 [0x8]))
         (lo_sum:DI (reg:DI 167)
                    (symbol_ref:DI ("f") [flags 0x86] <var_decl 0x7fa96ea1cc60 f>)
))

When handling (plus (plus (mult (a) (mem_shadd_constant)) (fp)) (C)) case,
the fp will be the lo_sum operand as above. We have assumption that the fp
is reg but actually not here. It will have ICE when building with option
--enable-checking=rtl.

This patch would like to fix it by adding the REG_P to ensure the operand
is a register. The test case gcc/testsuite/gcc.dg/pr109417.c covered this
fix when build with --enable-checking=rtl.

	PR target/111634

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_legitimize_address): Ensure
	object is a REG before extracting its' REGNO.

Signed-off-by: Pan Li <pan2.li@intel.com>