Don't generate superfluous mov instructions for move immediate on ARM64.
https://bugs.webkit.org/show_bug.cgi?id=136435

Patch by Akos Kiss <akiss@inf.u-szeged.hu> on 2014-09-03
Reviewed by Michael Saboff.

On ARM64, the size of an immediate operand for a mov instruction is 16
bits. Thus, a move immediate offlineasm instruction may potentially be
split up to several machine level instructions. The current
implementation always emits a mov for the least significant 16 bits of
the value. However, if any of the bits 63:16 are significant then the
first emitted mov already filled bits 15:0 with zeroes (or ones, for
negative values). So, if bits 15:0 of the value are all zeroes (or ones)
then the last mov does not need to be emitted.

* offlineasm/arm64.rb:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173205 268f45cc-cd09-0410-ab3c-d52691b4dbfc