Bug 861324 - Remove unneeded mDecoder null check after usage. r=joedrew
Bug 861543 - x86-64: Use movl instead of movabsq for 32-bits immediates. r=nbp

Make the implementation of mov in the x86-64 assembler use
movl instead of movabsq for loading immediates which fit in 32-bit
immediate fields.  This reduces the size of the instructions.

Also, change several places to call mov instead of using movq directly,
so that they can take advantage of this optimization.

--HG--
extra : rebase_source : 32f9a56a1cd7c794fe07f289bfaad9c2e9f5a55a
Backout 5f7a95537199 (bug 846122).