[RISCV][GISel] Remove source constraint from selectCopy and use it fo… (#67207)

…r G_ANYEXT/G_TRUNC.

AArch64, Mips, and ARM do not have source constraints in their version
of selectCopy. I'm assuming we don't need it either.

We weren't constraining the destination register of G_ANYEXT/G_TRUNC
previously, but we got away with it in our tests because all their users
constrained them as a use. When I removed the source constraint from
selectCopy this stopped working.