[turbofan] Use register for instruction operands when SameAsFirst is specified.
As the register allocator cannot reuse spill slots, SameAsFirst constraint means that we would have to do an expensive move to a differen spill slot if we choose to spill. Forcing the operand to a register is cheaper. In zlib, we get >10% speed-up for ia32, >25% for x64. BUG= R=bmeurer@chromium.org Review URL: https://codereview.chromium.org/650083003 git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24580 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
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