[Hexagon] Adjust latency between allocframe and the first store on stack

Allocframe and the following stores on the stack have a latency of 2 cycles
when not in the same packet. This happens because R29 is needed early by the
store instruction. Since one of such stores can be packetized along with
allocframe and use old value of R29, we can assign it 0 cycle latency
while leaving latency of other stores to the default value of 2 cycles.

Patch by Jyotsna Verma.

llvm-svn: 302034