RISC-V: Fix one range-loop-construct warning of avlprop

RISC-V: Fix one range-loop-construct warning of avlprop

This patch would like to fix one warning of avlprop as below.

../../gcc/config/riscv/riscv-avlprop.cc: In member function 'virtual
unsigned int pass_avlprop::execute(function*)':
../../gcc/config/riscv/riscv-avlprop.cc:346:23: error: loop variable
'candidate' creates a copy from type 'const std::pair<avlprop_type,
rtl_ssa::insn_info*>' [-Werror=range-loop-construct]
  346 |       for (const auto candidate : m_candidates)
      |                       ^~~~~~~~~
../../gcc/config/riscv/riscv-avlprop.cc:346:23: note: use reference type
to prevent copying
  346 |       for (const auto candidate : m_candidates)
      |                       ^~~~~~~~~
      |                       &

gcc/ChangeLog:

	* config/riscv/riscv-avlprop.cc (pass_avlprop::execute): Use
	reference type to prevent copying.

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