[TypePromotion] Don't treat bitcast as a Source

This removes BitCasts from isSource in Type Promotion, as I don't believe they
need to be treated as Sources. They will usually be from floats or hoisted
constants, where constants will be handled already.

This fixes #62513, but didn't otherwise cause any differences in the tests I
ran.

Differential Revision: https://reviews.llvm.org/D152112