[FastISel][X86] Add large code model support for materializing floating-point constants.

[FastISel][X86] Add large code model support for materializing floating-point constants.

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>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215595 91177308-0d34-0410-b5e6-96231b3b80d8