[llvm] [SLP] Use getRegUsageForType() rather than getNumberOfParts() when determining reduction width (PR #226277)

[Ryan Buchner via llvm-commits]

bababuck wrote:

> This seems to have some compile-time impact: https://llvm-compile-time-tracker.com/compare.php?from=0d049f14804d436b12d81846de00c45516541f0b&to=25485b3c4964ab704e7d4961b67ab2501fedbdbe&stat=instructions:u Is that expected?
> 
> In particular 7zip/DeflateEncoder.cpp is up by 50% with no size change.

Unexpected. Do you want me to revert, I won't be able to investigate further until the morning (PST).

https://github.com/llvm/llvm-project/pull/226277