[PowerPC] Materialize floats in the range [-16.0, 15.0].

Previous to this patch we only materialized 0.0 and all other floating point
values would be loaded from the TOC. This patch adds materialization for the
floating point values that can be represented as integers in [-16.0, 15.0].

For example we will now materialize 3.0 and -5.0 but not 4.7.

Reviewed By: nemanjai, lei, #powerpc

Differential Revision: https://reviews.llvm.org/D138844