[DAGCombine] Match a pattern where a wide type scalar value is stored by several narrow stores

This opportunity is found from spec 2017 557.xz_r. And it is used by the sha encrypt/decrypt. See sha-2/sha512.c

static void store64(u64 x, unsigned char* y)
{
    for(int i = 0; i != 8; ++i)
        y[i] = (x >> ((7-i) * 8)) & 255;
}

static u64 load64(const unsigned char* y)
{
    u64 res = 0;
    for(int i = 0; i != 8; ++i)
        res |= (u64)(y[i]) << ((7-i) * 8);
    return res;
}
The load64 has been implemented by https://reviews.llvm.org/D26149
This patch is trying to implement the store pattern.

Match a pattern where a wide type scalar value is stored by several narrow
stores. Fold 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;

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

>
*((i32)p) = BSWAP(val);

Differential Revision: https://reviews.llvm.org/D61843

llvm-svn: 362472