Reapply [FastISel][X86] Add large code model support for materializing...

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>.

llvm-svn: 216012