cmd/compile: fix debug info generation for loads from Phis
Apparently a LoadReg can take a Phi as its argument. The Phi has names
in the NamedValue table, so just read the Load's names from the Phi.
The example given, XORKeyStream in chacha20, is pretty complicated so I
didn't try to actually debug it and verify that the results are right.
But the debug logging looks reasonable, with the right names in the right
registers at the right times.