Reapply [FastISel][AArch64] Make use of the zero register when possible (r215591).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.

Original commit message:
This change materializes now the value "0" from the zero register.
The zero register can be folded by several instruction, so no
materialization is need at all.

Fixes <rdar://problem/17924413>.
Reapply [FastISel][X86] Emit more efficient instructions for integer constant materialization (r215593).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.

Original commit message:
This mostly affects the i64 value type, which always resulted in an 15byte
mobavsq instruction to materialize any constant. The custom code checks the
value of the immediate and tries to use a different and smaller mov
instruction when possible.

This fixes <rdar://problem/17420988>.
Reapply [FastISel][X86] Use XOR to materialize the "0" value (r215594).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.
Reapply [FastISel][X86] Add large code model support for materializing floating-point constants (r215595).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.

Original commit message:
In the large code model for X86 floating-point constants are placed in the
constant pool and materialized by loading from it. Since the constant pool
could be far away, a PC relative load might not work. Therefore we first
materialize the address of the constant pool with a movabsq and then load
from there the floating-point value.

Fixes <rdar://problem/17674628>.
Reapply [FastISel][AArch64] Add support for more addressing modes (r215597).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.

Original commit message:
FastISel didn't take much advantage of the different addressing modes available
to it on AArch64. This commit allows the ComputeAddress method to recognize more
addressing modes that allows shifts and sign-/zero-extensions to be folded into
the memory operation itself.

For Example:
  lsl x1, x1, #3     --> ldr x0, [x0, x1, lsl #3]
  ldr x0, [x0, x1]

  sxtw x1, w1
  lsl x1, x1, #3     --> ldr x0, [x0, x1, sxtw #3]
  ldr x0, [x0, x1]