Allocate SIMD regs for doubles on ARM
Turns out vixl already had gpr<->simd and simd<->mem instructions for
the uses cases we need. This was just a matter of making the SIMD
registers available to the register allocator and handling them in a few
places in codegen.
We're not allocating SIMD registers to hold Cells yet. vixl has no
support at all for 128-bit-wide SIMD registers, so I'll have to
implement that separately. This diff also doesn't add support for
passing doubles to builtins on ARM; that'll come next.
I also got rid of some now-useless isConst handling from ARM codegen.
Reviewed By: @edwinsmith
Differential Revision:
D1262941