[GlobalISel] Implement merging of stores of truncates.

This is a port of a combine which matches a pattern where a wide type scalar
value is stored by several narrow stores. It folds it into a single store or
a BSWAP and a store if the targets supports it.

Assuming little endian target:
 i8 *p = ...
 i32 val = ...
 p[0] = (val >> 0) & 0xFF;
 p[1] = (val >> 8) & 0xFF;
 p[2] = (val >> 16) & 0xFF;
 p[3] = (val >> 24) & 0xFF;
=>
 *((i32)p) = val;

On CTMark AArch64 -Os this results in a good amount of savings:

Program            before        after       diff
             SPASS 412792       412788       -0.0%
                kc 432528       432512       -0.0%
            lencod 430112       430096       -0.0%
  consumer-typeset 419156       419128       -0.0%
            bullet 475840       475752       -0.0%
        tramp3d-v4 367760       367628       -0.0%
          clamscan 383388       383204       -0.0%
    pairlocalalign 249764       249476       -0.1%
    7zip-benchmark 570100       568860       -0.2%
           sqlite3 287628       286920       -0.2%
Geomean difference                           -0.1%

Differential Revision: https://reviews.llvm.org/D109419