optimize Zicond conditional select cases.

When one of the two input operands is 0, ADD and IOR are functionally
equivalent.
ADD is slightly preferred over IOR because ADD has a higher likelihood
of being implemented as a compressed instruction when compared to IOR.
C.ADD uses the CR format with any of the 32 RVI registers availble,
while C.OR uses the CA format with limit to just 8 of them.

Conditional select, if zero case:
rd = (rc == 0) ? rs1 : rs2

before patch:

  czero.nez rd, rs1, rc
  czero.eqz rtmp, rs2, rc
  or rd, rd, rtmp

after patch:

  czero.eqz rd, rs1, rc
  czero.nez rtmp, rs2, rc
  add rd, rd, rtmp

Same trick applies for the conditional select, if non-zero case:
rd = (rc != 0) ? rs1 : rs2

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_expand_conditional_move):
	replace or with add when expanding zicond if possible.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zicond-prefer-add-to-or.c: New test.