[Sparc][NFC] Clean up SelectCC lowering

The ARM, BPF, MSP430, Sparc and Mips backends all use a similar code sequence 
for lowering SelectCC. As pointed out by @reames in D29937, this code isn't 
particularly clear and in most of these backends doesn't actually match the 
comments. This patch makes the code sequence clearer for the Sparc backend 
through better variable naming and more accurate comments (e.g. we are 
inserting triangle control flow, _not_ diamond). There is no functional 
change.

Differential Revision: https://reviews.llvm.org/D37194

llvm-svn: 312713