Hydrogen attempts to force representation changes on certain operations in order
to deoptimise on the change rather than the operation. However, these forced
changes are often unnecessary on 64-bit platforms, and cause poor code
generation, so this patch makes some of them conditional on whether it's
possible for deoptimisation to occur in the change.

On ARM64, this prevents sequences like:
                  ;;; <@46,#89> smi-tag
0x7ff282c7f050   144  lsl x4, x4, #32
                  ;;; <@48,#90> smi-untag
0x7ff282c7f054   148  asr x5, x4, #32
                  ;;; <@50,#31> mul-const-i-s
0x7ff282c7f058   152  lsl w6, w5, #3

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/303263010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21818 ce2b1a6d-e550-0410-aec6-3dcde31c8c00