Clean up use of encode/decode in the ARM integrated assembler.

Cleans up function names containing "decode" and "encode". Change
"encode" to represent the conversion of values into bit-sequences, as
they appear in instructions. Change "decode" to represent the extraction
of values from bit sequences.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1490383002 .