Add missing operand predicate, otherwise any RTX will match.

2021-08-06  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
    PR target/101797
    * config/i386/i386.md (cmove reg-to-reg move elimination peephole2s):
    Add general_gr_operand predicate to operand 3.

gcc/testsuite/
    PR target/101797
    * gcc.target/i386/pr101797.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to master.

Uros.