[NFC] Use more appropriate SmallVectorImpl::append call in...

[NFC] Use more appropriate SmallVectorImpl::append call in std::initializer_list SmallVector constructor

Since we are in constructor there is no need to perform redundant call to SmallVectorImpl::clear() inside assign function.
Although calling cheaper append function instead assign doesn't make any difference on optimized builds
(DSE does the job removing stores), we still save some cycles for debug binaries.

Differential Revision: https://reviews.llvm.org/D130361