[X86] Add test case to show failure to fold load with getmantss due to isel...

[X86] Add test case to show failure to fold load with getmantss due to isel pattern looking for Constant instead of TargetConstant

The intrinsic has an immarg so its gets created with a TargetConstant
instead of a Constant after r372338. The isel pattern was only
updated for the register form, but not the memory form.

llvm-svn: 372457