[Hexagon] Assume all extendable branches to be of size 8 in relaxation

[Hexagon] Assume all extendable branches to be of size 8 in relaxation

The branch relaxation pass collects sizes of all instructions at the
beginning, before any changes have been made. It then performs one pass
over all branches to see which ones need to be extended. It does not
account for the case when a previously valid branch becomes out-of-range
due to relaxing other branches.
This approach fixes this problem by assuming from the beginning that
all extendable branches have been extended. This may cause unneeded
relaxation in some cases, but avoids iteration and recomputing instruction
sizes.

llvm-svn: 328360