emit >>0 in HEAP8, for consistency with HEAP16|32
avoid unneeded |0 now that we >>0 HEAP8 accesses
remove unneeded |0 also in memcpy
Make bottom-up and top-down liveness propagation independent. This fixes Bug 1006301.
Split alloca liveness propagation into separate forward and backward phases.

This makes the code simpler, and avoids problems with the two directions
interfering with each other. This fixes asm2.test_sqlite and other failures.
1.18.0