[CodeGen] Emit more precise AssertZext/AssertSext nodes.
This is marginally helpful for removing redundant extensions, and the
code is easier to read, so it seems like an all-around win. In the new
test i8-phi-ext.ll, we used to emit an AssertSext i8; now we emit an
AssertZext i2, which allows the extension of the return value to be
eliminated.
Differential Revision: https://reviews.llvm.org/D49004
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336868
91177308-0d34-0410-b5e6-
96231b3b80d8
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