[PowerPC] Add prefix load pattern for fpext to v2f64

This patch adds a prefixed load pattern involving v2f32 fpext v2f64, where we
are dealing with a value with an offset that fits into a 34-bit signed immediate.
A reduced test case is also added to patch that tests the pattern, in which the
pattern is tested in the big endian CHECKs of the newly added test.

Differential Revision: https://reviews.llvm.org/D109887