AMDGPU: Simplify and improve sincos matching

The first trivial example I tried failed to merge due to the user scan
logic. Remove the complicated scan of users handling with distance
thresholds, with a same block restriction. The actual expansion of
sincos is basically the same size as sin or cos individually. Copy the
technique the generic optimization uses, which is to just use the
input instruction as the insert point or just insert at the start of
the entry block.

https://reviews.llvm.org/D156706