AMDGPU/GlobalISel: Don't constrain source register of VCC copies

This is a hack until I come up with a better way of dealing with the
pseudo-register banks used for boolean values. If the use instruction
constrains the register, the selector for the def instruction won't
see that the bank was VCC. A 1-bit SReg_32 is could ambiguously have
been SCCRegBank or VCCRegBank in wave32.

This is necessary to successfully select branches with and and/or/xor
condition.

llvm-svn: 366120