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[CGP] Split large data structres to sink more GEPs

Description

[CGP] Split large data structres to sink more GEPs

Accessing the members of a large data structures needs a lot of GEPs which
usually have large offsets due to the size of the underlying data structure. If
the offsets are too large to fit into the r+i addressing mode, these GEPs cannot
be sunk to their users' blocks and many extra registers are needed then to carry
the values of these GEPs.

This patch tries to split a large data struct starting from %base like the
following.

Before:
BB0:

%base     =

BB1:

%gep0     = gep %base, off0
%gep1     = gep %base, off1
%gep2     = gep %base, off2

BB2:

%load1    = load %gep0
%load2    = load %gep1
%load3    = load %gep2

After:
BB0:

%base     =
%new_base = gep %base, off0

BB1:

%new_gep0 = %new_base
%new_gep1 = gep %new_base, off1 - off0
%new_gep2 = gep %new_base, off2 - off0

BB2:

%load1    = load i32, i32* %new_gep0
%load2    = load i32, i32* %new_gep1
%load3    = load i32, i32* %new_gep2

In the above example, the struct is split into two parts. The first part still
starts from %base and the second part starts from %new_base. After the
splitting, %new_gep1 and %new_gep2 have smaller offsets and then can be sunk to
BB2 and folded into their users.

The algorithm to split data structure is simple and very similar to the work of
merging SExts. First, it collects GEPs that have large offsets when iterating
the blocks. Second, it splits the underlying data structures and updates the
collected GEPs to use smaller offsets.

Differential Revision: https://reviews.llvm.org/D42759

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