RISC-V: Eliminate redundant zero extension of minu/maxu operands

RISC-V: Eliminate redundant zero extension of minu/maxu operands

RV64 the following code:

  unsigned Min(unsigned a, unsigned b) {
      return a < b ? a : b;
  }

Compiles to:
  Min:
       zext.w  a1,a1
       zext.w  a0,a0
       minu    a0,a1,a0
       sext.w  a0,a0
       ret

This patch removes unnecessary zero extensions of minu/maxu operands.

gcc/ChangeLog:

	* config/riscv/bitmanip.md: Added expanders for minu/maxu instructions

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zbb-min-max-02.c: Updated scanning check.
	* gcc.target/riscv/zbb-min-max-03.c: New tests.