This avoids doing getConstantSym to avoid hitting the global
context's getConstantSym during emitIAS(), which may be desirable for
multi-threading, since each function's emitIAS() should be able to happen
on a separate thread.

The stringification is moved till later, so it still happens, just without
creating a constant relocatable w/ offset of 0.

This ends up tickling an issue where -O0 on 252.eon now gets 2x as many
page faults, and I'm not sure exactly why. This makes the overall time
higher, though emit time is lower.

When translating with -O2 # of page faults is about the same before/after,
so that oddness is restricted to O0.

Before this change, tweaking the slab size at O0 doesn't
seem to affect as drastically as 2x swings either.

To work around this, I turned the slab size of the assembler down to 32KB.

===

Move all the .L$type$poolid into a function (replacing getPoolEntryID).

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/837553009