Revert r372366 "Use getTargetConstant for BLENDI, and add a test to catch it."

This reverts commit 52621307.

Tests have been failing all night with

    [0/2] ACTION //llvm/test:check-llvm(//llvm/utils/gn/build/toolchain:unix)
    -- Testing: 33647 tests, 64 threads --
    Testing: 0 .. 10..
    UNRESOLVED: LLVM :: CodeGen/AMDGPU/GlobalISel/isel-blendi-gettargetconstant.ll (6943 of 33647)
    ******************** TEST 'LLVM :: CodeGen/AMDGPU/GlobalISel/isel-blendi-gettargetconstant.ll' FAILED ********************
    Test has no run line!
    ********************

Since there were other concerns on https://reviews.llvm.org/D67785,
I'm just reverting for now.

llvm-svn: 372383