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[RISCV] Improve 64-bit integer materialization for some cases.

Authored by craig.topper on Apr 1 2021, 8:51 AM.

Description

[RISCV] Improve 64-bit integer materialization for some cases.

This adds a new integer materialization strategy mainly targeted
at 64-bit constants like 0xffffffff where there are 32 or more trailing
ones with leading zeros. We can materialize these by using an addi -1
and srli to restore the leading zeros. This matches what gcc does.

I haven't limited to just these cases though. The implementation
here takes the constant, shifts out all the leading zeros and
shifts ones into the LSBs, creates the new sequence, adds an srli,
and checks if this is shorter than our original strategy.

I've separated the recursive portion into a standalone function
so I could append the new strategy outside of the recursion. Since
external users are no longer using the recursive function, I've
cleaned up the external interface to return the sequence instead of
taking a vector by reference.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D98821

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