optimize Zicond conditional select cases.

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.