Cache stack map encoding

Operations on CodeInfo and StackMap objects repeatedly read encoding
information from the MemoryRegion. Since these are 3-bit-loads of
values that never change, caching them can measurably reduce compile
times.

According to benchmarks, this patch saves 1-3% on armv7, 2-4% on x86,
and 0-1% on x64.

Change-Id: I46b197513601325d8bab562cc80100c00ec28a3b