cmd/compile: eliminate copy for static literals

cmd/compile: eliminate copy for static literals

*p = [5]byte{1,2,3,4,5}

First we allocate a global containing the RHS.  Then we copy
that global to a local stack variable, and then copy that local
stack variable to *p.  The intermediate copy is unnecessary.

Note that this only works if the RHS is completely constant.
If the code was:
*p = [5]byte{1,2,x,4,5}
this optimization doesn't apply as we have to construct the
RHS on the stack before copying it to *p.

Fixes #12841

Change-Id: I7cd0404ecc7a2d1750cbd8fe1222dba0fa44611f
Reviewed-on: https://go-review.googlesource.com/22192
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>