Do not allocate a large block for indexing.  Instead, allocate
for each temporary as they are seen.

In general, this will use less memory, if we consider that most
TBs do not touch every target register.  This also allows us to
allocate TempOptInfo for new temps created during optimization.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>