RISC-V: THEAD: Fix ICE caused by split optimizations for XTheadFMemIdx.

Due to the premature split optimizations for XTheadFMemIdx, GPR
is allocated when reload allocates registers, resulting in the
following insn.

(insn 66 21 64 5 (set (reg:DF 14 a4 [orig:136 <retval> ] [136])
        (mem:DF (plus:SI (reg/f:SI 15 a5 [141])
                (ashift:SI (reg/v:SI 10 a0 [orig:137 i ] [137])
                    (const_int 3 [0x3]))) [0  S8 A64])) 218 {*movdf_hardfloat_rv32}
     (nil))

Since we currently do not support adjustments to th_m_mir/th_m_miu,
which will trigger ICE. So it is recommended to place the split
optimizations after reload to ensure FPR when registers are allocated.

gcc/ChangeLog:

	* config/riscv/thead.md: Add limits for splits.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadfmemidx-medany.c: New test.