[Peephole] Rewrite copies to avoid cross register banks copies.

By definition copies across register banks are not coalescable. Still, it may be
possible to get rid of such a copy when the value is available in another
register of the same register file.
Consider the following example, where capital and lower letters denote different
register file:
b = copy A <-- cross-bank copy
...
C = copy b <-- cross-bank copy

This could have been optimized this way:
b = copy A  <-- cross-bank copy
...
C = copy A <-- same-bank copy

Note: b and C's definitions may be in different basic blocks.

This patch adds a peephole optimization that looks through a chain of copies
leading to a cross-bank copy and reuses a source that is on the same register
file if available.

This solution could also be used to get rid of some copies (e.g., A could have
been used instead of C). However, we do not do so because:
- It may over constrain the coloring of the source register for coalescing.
- The register allocator may not be able to find a nice split point for the
  longer live-range, leading to more spill.

<rdar://problem/14742333>