1. [RISCV] Pattern-match more vector-splatted constants (details)
Commit d85a198e85253b6b39d9b86eb7afd3332637bcbe by fraser
[RISCV] Pattern-match more vector-splatted constants

This patch extends the pattern-matching capability of vector-splatted
constants. When illegally-typed constants are legalized they are
canonically sign-extended to XLenVT. This preserves the sign and allows
us to match simm5. If they were zero-extended for whatever reason we'd
lose that ability: e.g. `(i8 -1) -> (XLenVT 255)` would not be matched
under the current logic.

To address this we first manually sign-extend the splatted constant from
the vector element type to int64_t. This preserves the semantics while
removing any implicitly-truncated bits.

The corresponding logic for uimm5 was not updated, the rationale being
that neither sign- nor zero-extending a legal uimm5 immediate should
change that (unless we expect actual "garbage" upper bits).

Reviewed By: craig.topper

Differential Revision:
The file was modifiedllvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp