Avoid unnecessary copy/load in EvalLoc() and LoadValue().

EvalLoc()/EvalLocWide() are used to prepare a register where
a value is subsequently stored, so they shouldn't copy the
old value to the new register for register class mismatch.

The only exception where we actually need a copy is
LoadValue()/LoadValueWide(), so we inline the old code that
makes the copy there. We also avoid loading inexpensive
constants when the value is already in the register.

Change-Id: I07519e9d4d9b3f7272233d196435f3035e4a3ca9