[PowerPC] Fix problems with large code model (PR17169).

Large code model on PPC64 requires creating and referencing TOC entries when
using the addis/ld form of addressing.  This was not being done in all cases.
The changes in this patch to PPCAsmPrinter::EmitInstruction() fix this.  Two
test cases are also modified to reflect this requirement.

Fast-isel was not creating correct code for loading floating-point constants
using large code model.  This also requires the addis/ld form of addressing.
Previously we were using the addis/lfd shortcut which is only applicable to
medium code model.  One test case is modified to reflect this requirement.