[Hexagon] Implement undoing .cur instructions in packetizer

The packetizer needs to convert .cur instruction to its regular form if
the use is not in the same packet as the .cur. The code in the packetizer
handles one type of .cur, which is the vector load case. This patch
updates the packetizer so that it can undo all the .cur instructions.
In the test case, the .cur is the 128B version, but there are also the
post-increment versions.

Patch by Brendon Cahoon.

llvm-svn: 302032